Have to take into account a lot of factors. Getting paid 2-3 million to move sounds good, but what are you giving up in additional tickets sales, concessions and parking fees at TDECU. In the end the school might get what, and additional 1-2 million. Also lost opportunity for season ticket sales if they just move big games to NRG. (I reduced my extra TCU tickets directly due to moving big games away from the home stadium - casual fans are not interested in time and effort for teams like Grambling, Southern or Arkansas State)
A full TDECU would create quite a good atmosphere in the stadium. I been to a few NFL stadiums and they generally lack the college vibe. Also, for on TV in regards to stadiums, the TV usually points at the field majority of game, is only when there are empty seats that anyone notices. A full stadium looks good on TV whether it is 40,000 or 80,000.