Another point. I stumble across so many airlines that were absorbed by / purchased by legacies and Southwest that I never knew existed.
You have a better chance of being purchase by a legacy (and never having to interview again!) if you are at a ULCC than if you are at a regional. It just happened at Virgin. A legacy simply won't buy a regional and add them to the seniority list when all they have to to is shift fly from a mature regional to one with a lower pilot cost. There are still hundreds of 50 seaters they want gone too.

However, I would not be surprised if a legacy purchased a ULCC before it got to big. They would get:

Close to 100 Airbus aircraft with average age of 5.6 years.
Orders for several more which could replace their aging mad dogs and 757s.
1500 typed Airbus pilots
An operation that they no longer have to compete against.
An operation that is the most profitable in the USA.
An operation with extensive international flying.
STRANGER things have happened.
I cannot predict the future, but I could see a ULCC merger, or a purchase by a legacy within the next 10 years.