Well I hate to be the Negative Nancy on this thread, but frankly I'm shocked at the number of people that are salivating over F9/YX.
There is one major difference that separates US from the (soon to be) 3 biggest legacies: Hubs in the top US metropolitan areas - THAT is what US needs.
F9/YX offers none of that by any stretch of the imagination, but what they do offer is hubs in two of the most LCC-infested cities in America for their size. While US could use a midwest hub, YX's MKE operation is not the answer. FL's operation is rapidly increasing, eroding any advantage that YX once had. True, DEN would be a marignally better hub from a geographic standpoint than PHX, but there is absolutely no need for both. And that of course says nothing about the 3-way slugfest that is going on there right now. We saw how that outcome worked out in the 1980s and while the overall airline environment is not the same 20+ years later, I don't expect the outcome to be any different.
I wish the folks at F9/YX the best, as they even more so than US, are underdogs in this industry. However, they have the challenges of US (i.e, LCC competition, mid-sized hubs, etc) without any of the strengths (i.e, large portfolio of slots, international presence, hubs that aren't completely trashed by LCCs). Sure they have the advantage of using cash from their regional arrangements to fund F9/YX, but if F9/YX proves to be a disaster, the investors won't continue to put up with that for an eternity.