The only reason not to do so is the vanishingly rare chance that democrats turn a red state, which is basically the exact same argument they use to tell people to vote blue no matter who, but more compelling because it's not typical genocidal am*rican politicians they're voting for.
What are they going to do? Tell you they want to vote for a war criminal on principle? The positive political pressure of having a somewhat popular socialist party far outweighs the small chance democrats could have won (and this is even from their perspective, that the democrats winning would fix everything somehow)
have you met an American liberal??
a lot of US liberals who backed Bernie were actually serious about the 'or bust' and jumped ship from the Democrats after. I've worked with, organized with, and even been roommates with some of them, amounting to a pretty good number just in my personal bubbles. A bunch of them are even communists now. And even more have been going that route since, including hugely now over this genocide in Palestine, which is seeing also mass sustained protests even outside of the Democratic Convention, which is happening alongside many defections. Liberalism is in the water from birth; most US communists on HB were once US liberals. It is the default slate before any other input. Saying liberals can never be convinced is demonstrably false because you're speaking contradictory negations about the literal experiences of many people on this very web site. It also doesn't serve the expanding of class consciousness or construction of socialist movement building to hold and push that idea, which I've seen many times on this site, which makes me laugh when I know it is coming from a US person who was, in all likelihood, a liberal just a few years before making such comments.
It can be done. Don't expect all the brain worms to leave though. New comrades and old will all make errors and as long as someone is on the side of the angels we should be kind and give space where possible. Heck it is in theory possible that even I, the one true leftist, will make an ideological error.
You can pretty much convert any liberal or conservative that’s willing to learn from books as opposed to podcasts and memes and out of context graphs. It’s just annoying that there aren’t many of those people around.
But yeah, liberals can be converted. I just have less patience for it because they’re usually incredibly condescending about shit they have little to no clue about, and it takes pushing through that arrogance to actually convince them to read about these things in depth enough to actually understand them.