A big part of the Democrats’ problem is having attitudes like this, whereas no Republican would dream of suggesting that any particular voter could not be converted to movement conservatism. (Which is why you get frequent inanities like the assumption that Larry Elder’s campaign could win California; Republicans have never written off that state, or any other one.)
The problem with triaging is that eventually you’ve triaged >50% of the electorate.
I think part of the problem is that Democrats and the left more generally have bought their own bullsh-t to the point that they believe anyone who voted Republican last time is practically irredeemable. It's why you hear them talk about turnout or waiting for trends to save them way more than you ever hear about any kind of persuasion. It's like they don't
want to win if it means selling out to the people they call evil. They'd rather lose and let a Republican appoint more judges than tone down some of the identity politics crap or take a mostly performative stand against illegal immigration. They don't want to win over those people.