It's probably thier best chance, actually.
They're nearly certain to lose the Senate after midterms, so trying to replace him later will have to run the gauntlet of a GOP senate that can just keep rejecting nominees until Biden caves and nominates someone they approve of. The senate probably can't force him to put a scalia-esque constitutionalist up on the court, but they might be able to force him to put another Kennedy on the bench.
And if they try and wait until later, when they manage to gain the senate again, that's going to be a few years yet, particularly because they must know that the odds of them still having the WH in 2024 are poor and Breyer will have to hang on until 2028. They're not going to take that sort of risk again, not after Barrett got on to replace Ginsburg.
Filling the SC with political hacks won't weaken it, the SC is at it's most dangerous when they stop being a court and start being an unaccountable super-legislature. The way to limit the SC is to have justices that will stay in their lane.