Ok sorry if this is a stupid question, but why not let them?
It seems like the biggest concern is a humanitarian one - that sane people who remain in Texas will be worse off - but it significantly improves that same problem for everyone else in the union. And a war would possibly be even worse for sane Texans. It would be cheaper to subsidize relocation costs.
I just don't get why we'd fight to keep them, or pretty much any state these days. They don't have slavery, and there's nothing huge that we were right about to change.