Instead they just tell leftists "see we didn't need ya'll after all!! We shouldn't waste time trying to win you over"

I've never heard or seen that, but that might just be the bubble phenomenon, so I'll take your word for it. And I agree, that's fucking stupid. For one, if I narrowly made it, I'd want to increase my margin for the next time. Secondly, it probably overlooks all the leftists that did compromise to avoid the greater evil, so "we didn't need ya'll" is a good way to discount their value and discourage future cooperation.

Somehow leftists are both too few in number to bother winning over

I suspect for the party leadership, it's more of a "too expensive to win over" thing.

but enough in number to swing an election when Dems lose

I also suspect that the lion's share of the non-voters aren't even leftists, or at least not consciously, but idiots in the original Ancient Greek sense: People only concerned with their own business instead of participating in the matters of the polis (= politics). It's obviously hard to tell where they would lean if they did participate, but abstaining entirely is irresponsible, second only to actually voting for the people that have been quite blunt and open about their intention to take away your vote.

I still maintain that the priority of damage control should scale with the stakes and consciously accepting the risk of the objectively worse outcome is short-sighted. I understand and appreciate the signalling value of a third-party vote, and I definitely agree that the blame for their defeat should be on the Dems for being so bloody useless. I just disagree that its long-term contribution to shifting the political balance outweighs the short-term destructive effects of a potential R victory at the federal level, particularly the last few elections.

But it's a lot better than complacency. You've got a spine and you're showing it, and I respect that. Here's to hoping that all my concerns end up being wrong - I'd rather be a pessimist than a realist.