Smart for the Big 12 isn't necessarily the path to the biggest payout for these two schools. It could be, but it would take patience and nurturing of at least four new members. It's easier for Texas and OU to say it's not worth the wait when they can get an immediate payout by one of the top two conferences (SEC or Big Ten). Today, the Big 12 is getting money that puts them fairly firmly in the number three position and the ACC is going to get mucho dinero out of their new network that could put them at or above the Big 12.
To me, an objection to extending the GOR with adding new members is an indication of intent. This is my opinion and I know everyone has ideas or theories so my two cents...
I believe OU wants to keep options open because they have legitimate shot at landing in the Big Ten, along with Kansas to keep the contiguous states clause intact. I further believe, OU would prefer to make the Big 12 viable and are willing to expand in that effort, but aren't willing to place all of their eggs in the Big 12 basket. The SEC is also very viable for OU and OSU.
Texas is a different story. I don't think the Big Ten adds them along with OU and Kansas. I don't think the SEC would take them due to TA&M, though Texas must know something or be in contact with someone. The ACC would make less sense with super extended travel. Do they really want to send their Olympic sports teams to N Carolina, Miami Fla, and Syracuse and all point in between? Independent is a fading option (even ND formed an affiliation).
At the end of the day, smart for the Big 12 could be the best path for OU and Texas, but Texas doesn't see it that way and OU thinks of it as only a "could be". I hope they give it a shot and then after adding UC & UH for all sports, and BYU & UConn for football only, we collectively put the Big 12 on par with the SEC and Big Ten in terms of value (both revenue and competitive).