It would be a proportionate and appropriate response, and hopefully restore an equilibrium in response to a constitutional crisis provoked by McConnell.
It would be a disaster and escalate beyond reason. What one side does, the other side will do harder.
Far from a disaster, it will be something approaching mandatory to restore faith in the institution. The disaster already came. But because this response would come in the future and the disaster happened in the past, some people have the psychological reaction of thinking that a new thing is unacceptable while uncritically accepting bad things that have already happened, which is called shifting baseline syndrome.
The Republicans already take everything to 11, as a matter of stated policy, since 2008, regardless of whether the Democrats go to 10.
Note that Trump and McConnell are rushing to fill the Supreme Court, despite 4 years of leaving about half the government posts vacant.
If that were true they had plenty of time to pack the court themselves, but they haven’t.
It is correct to say that they have had plenty of time to provoke a constitutional crisis. It is also correct to note that they could have resorted to abusive procedure even worse than the one they already did. Which is like saying they created a category four hurricane instead of a category five.
Not sure what the calculus is. If it’s unfilled, Biden will fill it. If it’s filled, he’ll expand the court. Not sure if that also depends on a senate flip, but I have a hard time believing the senate doesn’t flip during a Biden win.