Schrödinger’s Leftist: simultaneously an irrelevant tiny minority with no power, and crucially important to Biden winning, being wholly responsible if Trump wins.

No one is ever wholly responsible if the right wins, obviously. Wins are always based on a broad spectrum coalition, whereas losses are based on a coalition just barely failing. Modern US presidential elections are always close. Still, that doesn't mean a potential coalition member gets to dictate coalition policy, especially when they're on the extremes.

Unconditionally voting Blue means I have no influence at all.

Barring the demise of FPTP, you will never get what you want. Instead, politicians just learn you will never vote for them and they should look to more conservative constituencies. That, of course, means policies you don't like.

No, it really isn’t.

It's called compromise, and yes it is how a functioning democracy works.