I agree that on the surface it's a little confusing why Texas would join the SEC just in terms of making the playoff becomes more challenging. Here's how I rationalize the move.
First off, don't underestimate the impulse for the rich to get richer. There's never enough money and power to satisfy leaders in college sports. If there's more money to make, they'll find a way to make it.
Second, the Big 12 brand is in decay when it comes to college football. The league doesn't produce NFL talent like the other power leagues. It doesn't produce championship contenders, especially outside of Norman, Oklahoma. The longer Texas and OU stays in the Big 12, the longer they risk becoming part of an irrelevant home for college football. We're to the point where the runner-up and third-best team in the SEC can be just as appealing, if not more, than the Big 12 champion when it comes to the college football playoff. Surely Texas and OU could sense that - and it's better for their vitality - financially and prestige-wise - to swallow their pride and join the best conference, even if it means sacrificing some power and influence.
Texas boosters don't envision 8-4. Yes, it happens - and usually worse here lately - but that's not how they think. They'll go into the SEC expecting to challenge Alabama for supremacy. Naive? Yes. But Texas will always have a ceiling as high or higher than any program in the country.