You don’t have to replace them next election with a far left candidate, just one that won’t betray the party like those two shit-heads. You run the risk of losing the seat to the GOP but it was half GOP anyways and its worth it to maintain party discipline. Kick two senators out and no other senator is going to risk their career disobeying the party.
Also what this utter nonesense about maintaining the filibuster? It can be removed with a simple majority and the GOP does so whenever they have that majority. Its been that way for decades. Saying “It’d be nice if the GOP kept the filibuster when they were in power so we will keep it when we’re in power.” is absolute bullshit. Democrats aren’t naiive idealists, they just want excuses to not do what their voters want.