Southwest's higher fares has almost nothing to do with the mergers and less competition. Over the last few years most carriers raised their fees to offset the extreme increase in fuel costs. Southwest was able to keep their fares lower during most of that time because it had excellent oil futures contracts for cheap fuel. Those contracts expired and Southwest could no longer avoid the effects of higher fuel costs. That is why Southwest's fares are not as low anymore.
DJ is correct, and we should NOT consider WN (Southwest Airline) fare increases in relation to the legacy carriers.
In the 2000s, WN hedged their fuel rate correctly, and they used this savings (and other factors) to artificially keep fares below the average for a decade. Only relatively recently have we seen their airfares increase to meet/exceed the legacy carriers' pricing.
In addition, WN has several expensive labor contracts that are also causing a significant effect on their current pricing model. A TV news story (I can search Google if someone demands it, or they can, too

) indicated that their senior management team would give the union team members almost all their concession points in an effort to keep morale high among their crews. These expenses are also coming to fruition.
Since WN does not operate a standard hub & spoke model, they seem to price legs separately, which could add to the price of an individual ticket depending on destination and stops. In other words, a non-stop would cost less than a direct flight, which would cost less than a non-direct (plane change).
Add all of this and other factors to the mix (primarily leisure travelers vs business, no interline agreements, frequent flyer program revenue/expenses, no charge baggage, no change fees, entry into mainline airports vs ancillary regional airfields, others) and WN's expenses have increased for several different reasons than those of the legacy carriers.
On another note, I would say the current consolidation of legacy airlines effect business travelers more than leisure travelers due to inclusion of international travel, their extensive frequent flyer program, first and business class products, and route network. While WN does have a cadre of business travelers, their model appears to be centered around catering to the domestic leisure traveler, which doesn't really care about class of service vs ticket price nor the international route network. As such, I would premise the consolidation has minimal impact on WN's clientele and flight pricing.