MIAMI — Last December, Southwest Airlines finally wrapped up its lengthy integration process with AirTran Airways, fully drawing the merger to a close. As “Citrus One” took to the skies bound for Tampa, and as American’s Flight 1939 hit the ground in Philadelphia just last month, it seemed to signal the conclusion of the merger mania played through the industry during the last decade.

Slowly but surely, consolidation has whittled down the playing field, leaving only four major carriers standing. Collectively, the “Big Four” – American, Delta, United, and Southwest – now dominate over three-quarters of the domestic air travel market. With just four players in firm control, and with each of them having already found a partner, it would appear merger era in the airline industry has come for a landing.

However, some recent talk indicates another merger may lurk on the horizon, continuing the game of merger mania. Specifically, rumors continue to swirl that Southwest Airlines, fresh off absorbing AirTran under its wing, may have its eyes on acquiring another carrier to fuel its growth. According to the Chicago Business Journal, Frontier and JetBlue stand as the two primary targets.

But despite the speculation, Southwest is unlikely to engage in another merger in the near future. Any merger proposal would face significant headwinds in gaining approval from the Department of Justice (DOJ). Furthermore, neither company represents a logical target for Southwest, unlike AirTran which fit cleanly with the airline’s underlying structure. While more consolidation may lie down the road within the broader airline industry, don’t expect a move from Southwest Airlines anytime soon.

Firstly, receiving takeoff clearance from the Justice Department marks a major, possibly insurmountable hurdle for any sizable merger. The recent wave of airline consolidation includes the United-Continental, Delta-Northwest, Southwest-AirTran, and American-US Airways unions. And as each domino fell, the DOJ displayed more reluctance to grant their stamp of approval. The DOJ raised firm objections to the most recent pact between American and US Airways before ultimately allowing it to proceed, albeit given some hefty concessions. One could reasonably imagine the next merger to hit their desk would face even more intense scrutiny, and quite possibly not obtain the green light.

During the era of industry turmoil that drove each and every legacy carrier into bankruptcy, the airlines always maintained a powerful card up their sleeves. Ridden with financial troubles, the airlines could capably claim that merging represented a necessary step to get back on track. Not only would consolidation yield a more lean operation in a better position to turn a profit, they argued, but consumers would also reap the benefits of a financially healthier sector. Fewer airlines competing more forcefully for passengers is better than a pool of insolvent ones.

But with the industry rejuvenated and the airlines raking in record profits, the argument that mergers would offer a public benefit no longer stands. Furthermore, the DOJ has expressed the sentiment that the current playing field, characterized predominantly by four major carriers and peppered by a number of low-cost competitors, marks the most consolidated it is willing to go. It seems highly unlikely the Justice Department, in these days of gushing profits, would willingly approve another merger of any sort, and certainly not one designed to bulk up one of the industry titans.

Even putting aside the questionable feasibility of another merger, both Frontier and JetBlue represent illogical targets for Southwest Airlines. For one, the incompatibility of the airlines’ fleets would pose a significant, perhaps the largest, stumbling block.

Southwest operates an exclusively Boeing 737 fleet, which contributes to keeping a lid on costs. Both Frontier and JetBlue, however, field a primarily Airbus-driven lineup. Merging with either of those two companies would require a significant deviation on the behalf of Southwest from its traditional strategy.

Even AirTran only operated two aircraft models before its pact with Southwest: the Boeing 737 and the Boeing 717, a much less incongruous fleet than what Frontier or JetBlue would have to offer. And although Southwest entered the merger with the original intention of flying both aircraft, it quickly changed course, making the call to lease all of its Boeing 717’s to Delta Air Lines. Clearly, Southwest’s management still prioritizes the advantages which stem from a homogenous fleet, and there is no reason to think they’ll be any more receptive to integrating an army of Airbus-produced planes into the airline’s fleet.

Maybe Alaska Airlines would fit more nicely with Southwest. But there are no indications that Alaska, currently embroiled in a fiery battle with Delta, is on the market.

Furthermore, neither Frontier nor JetBlue offers Southwest much in the way of strategic value. Frontier, for one, employs a wholly different business model than Southwest. As a carrier revolving almost entirely around the leisure traveler, Frontier would fail to build on the customer base that Southwest eyes.

Recently, Southwest has adopted a number of practices designed to augment its business travel numbers. CEO Gary Kelly indicated to Forbes a desire to scale up Southwest’s presence in the business travel market, inching up the airline’s proportion of business fliers from 35% to 40%. The legacy airlines typically exhibit about a “fifty-fifty ratio,” a figure to which Southwest wants to move closer.

As an ultra-low-cost carrier (ULCC), Frontier would hardly support Southwest’s pursuit of the business traveler, and would actually apply downward pressure to unit revenues. Frontier relies heavily on the cost-conscious leisure segment to fill its planes – an audience into which Southwest taps but does not depend on as exclusively. Certainly, boosting its unit revenue would represent a primary goal for Southwest in any future merger, an aim to which Frontier would poorly reinforce.

JetBlue would appear to better connect with Southwest’s long-term goals in this regard. It is situated similarly to Southwest, a “hybrid” carrier positioned in somewhat of a middle ground between the approaches adopted by the legacy carries and the ULCC’s. But even with a more favorable customer base, the advantages of a Southwest-JetBlue marriage appear less obvious than with the AirTran acquisition. Gaining immediate and strong access to the Atlanta market, one of the most important nationally for business travel, signified a clear plus in Southwest’s deal with AirTran.

AirTran filled a gaping void for Southwest. But while JetBlue would certainly add points to Southwest’s overall market share, it would inject much less strategic value. Yet the costs of any potential merger, both tangible and intangible, would run sky high.

Along with a list of benefits, mergers impose a number of costs on airlines. In addition to the steep financial price tag, Southwest would want to seriously consider the possible impact on the airline’s traditionally prized culture. While AirTran’s internal culture at least roughly matched that at Southwest, the environment at JetBlue meshes a little less tightly. Frontier stands even more starkly in contrast. A cultural rift could easily undermine the historically excellent customer service Southwest has boasted – one of the airline’s most valuable assets – and management should not and likely will not ignore these less visible costs.

Many within the industry expecting a Southwest merger point to management’s recent hastiness to sign new labor contracts with its front-line workers as evidence. But merging might even further jeopardize already deteriorated labor relations within the airline. Integrating workforces and blending seniority lists historically represents a cumbersome aspect of any union, and wields the potential to create tension for years to come. Former US Airways (now American) employees know this too well after the airline’s prior merger with America West, one laced with labor trouble. It seems a little misguided to say that Southwest’s management wants to quell internal labor unrest just to enter into another labor fight.

The benefits of a merger with Frontier or JetBlue simply do not match the costs. Conceivably, the game of merger mania may not have yet reached its end in the airline industry. Bill Franke himself, chairman of Frontier Airlines, hinted as such previously at an airline economics conference: “don’t we all wonder just how long it will take before the three to five lower-cost airlines in the States will consolidate?”

But another merger probably isn’t lingering on the frontier for Southwest. Don’t expect Southwest Airlines to show Frontier or JetBlue any LUV in the near future.