They'll do it anyway if they get back in power and it benefits them. They're also threatening to eliminate the filibuster if the GOP fills the seat. It's a hollow threat, they're trying to bluff the GOP into not filling the seat but you can guarantee that whether we fill it or not they're still going to try and push through these measures in a power grab if they get back into power.
The GOP would be the ultimate suckers to fall for it. There's no such thing as good will with democrats. If the GOP tries to placate these zealots by not filling the seat then they'll still eliminate the filibuster and try to stack the court if they get back into power. Maybe we'll get a reacharound in the process as a gesture of thanks.
They're going to do everything they can to consolidate power if they get voted back in, that's a given. We MUST get that seat filled before that can happen. If we don't then we'll end up with another Ginsburg on the court, no filibuster, and they'll still try to stack the court to increase their margin.
Didnt the SCOTUS already decide that the Court cannot be expanded? IIRC, FDR tried it and got shot down.