Even in a situation where Dems gain the AL seat, they'd still need to hold every seat they have in 2018, plus pickup NV/AZ. So unless the GOP dismantles to a point where they can't even defeat Claire McCaskill, they'd still have the senate.
1.) I agree that they will beat McCaskill, but she could always retire and a more electable Democrat could win the nomination. Unlikely, but possible.
2.) Even with that in mind, why would they voluntarily give away a Senate seat to the Democrats? Having 52 instead of 51 seats makes a big difference when it comes to passing legislation, obviously.