My theory is that because the US has been shifting right for decades, parts of the US leftwing has to some extent resigned itself to thought policing and arguin

My theory is that because the US has been shifting right for decades, parts of the US leftwing has to some extent resigned itself to thought policing and arguing semantics instead of fighting for actual policy changes. When you can't fix the big problems, find some small problem that you can focus on instead. If the US had a leftwing party that occasionally got in power (instead of a two-party system with a centrist and a rightwing party) then lefties would probably spend their effort on making that happen instead.