I doubt that any of them would ever come back to the Big XII. They are in conferences that give them some security and money. The Big XII is a risk until after the next expansion round.
However, outside of the money, they would be better off in the Big XII. Outside of Missouri's first year none of the four have really competitive in their new conference. Nebraska has probably lost the most from a pure football standpoint. Missouri and A&M aren't close to being competitive and may never be.
Colorado and Nebraska lusted over the Pac 12 and the Big Ten for decades and I'm sure they are happy. I don't think the Huskers have much chance in getting back to the great teams they had in the Big 8 and Big 12. The other three never really had consistent success over the long term.
If, we can keep four or five teams really competitive this conference can survive. It won't survive as Oklahoma and the nine dwarfs.