This is kind of building up/actually happening. The problem for them was that the rustbelt flipped before the sunbelt did. But I think they could maintain this coalition for a few cycles before new divisions in the electorate occur. They could have a huge margin in the house with a lot of seats on the two coasts + Illinois. Plus a small margin in the senate. It would probably happen all at once. Basically like what happened in Virginia. They could have a sustainable majority based on just urban/suburban states and districts the way Democrats do in Virginia without any real rural support at all. Which means they could just push through their agenda (especially on social issues) without fracture.
There's one problem with this theory: urban/suburban states have the Senate somewhat weighted against them. Given how many pro-filibuster Senate Democrats remain, they are not going to be able to push through their agenda without fracture for a long time.
Not who you were responding to but, given how many Senate Republicans said they were 'pro-filibuster' until the time came to actually put up or shut up, I'm not so quick to give "pro-filibuster Senate Democrats" the benefit of the doubt in what they're saying now being what will end up reflected in their inevitable vote to further limit or outright end the filibuster.
McConnell maintains a tight grip on his caucus and ending the filibuster for short-term political gain follows a long, Republican arc towards the centralisation and amassing of power. This doesn't hold as true for the Democrats (in part because pro-filibuster Senate Dems can run in primaries on Compromise/Bipartisanship and still get some credit for it), and Schumer has less sway over his caucus' moderate heroes.
I think there will eventually be filibuster reform, but it'll be an expansion of budget reconciliation rather than the end of the legislative filibuster for non-economic issues.
