Common fallacy in my opinion. Why does everyone assume that just because one airline sucks up another that you have to change operations? If Allegiant has a proven track record of making money, an airline like Spirit buys it up (or vice versa) and keeps everything the way it is. You can hub and spoke in some areas of the operation and out and backs for other parts that makes sense for profitability. Quite frankly I don't know why in the future Allegiant doesn't add hub and spoke where it makes sense but keep all the out and backs the way they are, its a win-win.
Besides all that, both Frontier and Spirit are doing just what Allegiant is doing for the most part or going more and more so in that direction. It would just make a bigger airline doing the same thing.
It will eventually happen. Airline mergers are to make a windfall for the big wigs, not help the consumer or employee groups. And you can count on Maury Gallagher make his big $$ move when he does his swan song on the way out. Count on it.