Texas State will not leave the playoff classification unless it has a commitment from a conference that is not named the Sun Belt. I think realistically they would want to join the western division of CUSA. The WAC is too much travel for them to afford and the Sun Belt would afford them less recruiting opportunities than the Southland does right now.
UTSA is still 5 years out from adding football. They would have to play in the playoff classification for 5 years past that to get their feet on the ground. But yes, once they have those in place and if they have the support of the city at that time, you could see them moving into the bowl classification.
Overall I see this less about where they want their football to play and more what conference they want to play in.
I see it a little differently. I think Texas State probably better have expressed an interest in moving up to the NCAA before the 4 year moritorium on new applications was announced. It seems likely that UTSA is one of the 20 teams and barring UT/the legislature shutting down the university and the Student body's desire to play I-A, they will probably be in the FBS by 2016 or so. They have a great media market that they alone own. No NFL competition. The only game in town. If they can pull even 25K to their games they could easily find their way into CUSA.
Now take a look at Texas State's situation at that point. With UTSA in the FBS, Texas St. would at that point almost certainly want to move up, but at that point the dynamics change entirely. Now you have a school wedged between UT and UTSA trying to jump to FBS and presumably challenge both schools for recruiting. Do you honestly think UTSA at that point is going to strongly argue for the admission of Texas St. into CUSA knowing that it will hurt their recruiting? Frankly moving up would seem a very difficult proposition with as many alumni as UTSA and UT have kicking around. Good luck getting that by the legislature.
Secondly, The Austin/San Marcos Market is nice, but not overwhelming. I don't thing CUSA is drooling to add it. Plus, Texas St. is a distant #2 in terms of fans in their local market. That screams Sunbelt, which Texas St. will not accept, creating more time in FCS.
Texas St.'s best option is to jump up at the same time as UTSA. If the two schools move up together and commit to staying in the same conference, they can have a ton of leverage getting into a league.
Personally, I think financially it is in the state of Texas's best interest to have our 5 regional giants with I-A aspirations (UNT, UTEP, Houston, Texas St., and UTEP) together in a conference. Maybe with Memphis and possibly LA Tech.
(I didn't mention one of the Texas regional giants. UT-Arlington is effectively "Football dead" with the new Dallas Cowboys Stadium going in in Arlington. They would be smart add as a I-AAA non-football member of the new conference --- New SWC?.)
That leverages the large alumni bases and all of the big media markets in the state to pull in a similar but more advanced school out of a disfunctional conference (Memphis) and La Tech (a school that isn't in the shreveport Market, but has as much of a claim on that market as any school. Add that to their Native Monroe Market and it isn't awful.). Those 7 schools would have a pretty good TV deal split between few mouths. I think they could offer Memphis a better share than CUSA can as currently constructed.