Global annihilation is a much greater risk of failing to check Russian aggression now than it is of checking it; the details may be different, but the same reason not checking Hitler at Poland would have made things worse (and that not doing so at the Sudetenland did make things worse) applies.

Sure, the potential cost of getting it wrong is even higher, but the same answer is wrong for the same reasons.

“Letting Ukraine do all the (overt, at least) fighting but flooding them with arms and intel will be sufficient to check Russia” is, OTOH, somewhere between Sudetenland and Poland as a response, and it's a very high stakes gamble.