I think if the border is a fraction of the crisis Republicans make it out to be, you take what you can get now and come back to it with Trump in the White House and/or Senate control. Both of those things are in play in November. Instead, you kill it and give the Democrats (you know, the dumbest people in the world, according to Republicans) a political win in which the GOP gets nothing.
If my house is on fire, and someone brings me a garden hose, I don’t say, “Nah. I’m good. Something better might come along eventually.”