These people have jobs they don't want to lose.
They are afraid if they oppose Trump, they might lose their job. He might endorse their primary opponent, and that's all she wrote. This is where his power comes from. He doesn't control the entire GOP, every conservative voter isn't beholden to him. But if he backs your opponent, the smaller group who DO blindly back him can win elections.
Mitch doesn't have this fear.
You asking if the 79 year old minority leader who has no chance of being primaried will survive isn't interesting, because it's already answered.
Pointing to Mitch and asking what about him, that's where words like disingenuous start getting used, because it's pretty clear he's not a typical Republican, and few politicians of any kind enjoy his current job security.
On the other hand, it'd be nice to be able to find something interesting with how he's able to go against the trend without it being "disingenuous" or somehow in bad faith. But unfortunately, that's how the forum works. I get it.