I don’t live in Texas, but I believe all of the Texas teams you have mentioned had down cycles recently. I imagine their fanbase survived those cycles.
As HRE mentions above, all the teams in the new SEC won't be terrible at once. The conference is very cyclical. There are probably 7/8 teams teams in the SEC who think they should win the conference title. All of them have competed for the conference (and even a national championship) in the past 20 years and all have also had bad seasons in that same time frame.
Unlike your perception (i assume biased towards SU), if it gets really bad for a while, the programs won't sit there and do nothing. Even in the ACC, FSU pulled a quick decision on Taggart (with the help of a very small group of boosters), Miami is stepping up with significant investment, etc.
Back to a Texas school (i looked this one out to make sure my memory was correct). Texas A&M fired Sumlin due to the downward trajectory after entering the SEC, despite his worst season of 7-6. Their fanbase seems back and louder than ever (based on media coverage)
Speaking of your reply to "football is a religion in Texas", that appears to actually be anecdotal. I think that saying is derived from Texas High School Football. NCAA Football is not the most popular sport in Texas (i know that might be hard to believe based on your said experience). NFL or MLB is, depending on source.