Theirs. The real argument is the GOP will do whatever they need to do to win a news cycle, so it's dead already. We just don't have the timestamp.
There might be a possibility they don't nuke it. But if they need to they will for something terrible. Every time something has been in their way the last 15 years, they broke precedent, and norms and pushed partisanship to get the result they wanted.
Nothing has changed, and to expect rationality is just insane at this point.
Democrats need to draw their line and hold. You only muddy the waters and provide cover by wishfully thinking DC is operating on the same rules it did 20 years ago.
i'm not saying it's likely, but republicans aren't insane. they operate based entirely on self-interest and long term policy goals like political parties should, and in that sense democrats are the aberration.
if republicans think they're likely to lose the senate they'd probably consider keeping the filibuster intact. but as i said, it's unlikely.