The Democrats have cobbled together a coalition of identity groups that actually have diverging interests, and don't really like each other all that much. As both parties are in a constant state of evolution, we've now reached a state where the Democrats have run their margins up hugely in some areas, while losing vast swaths of the country. That is a recipe for political disaster due to the structure of the US and State governments. Our system is designed to force everyone toward some sort of consensus and compromise, or to just shut down when that isn't possible.
Meanwhile, the Republicans are working hard at peeling off at least parts of each of the Democrats' constituent groups.
Thinking that parts of the country are irrelevant is beyond foolish. That has resulted in a vast shrinkage of the "purple" in electoral maps over about a decade, while "blue" areas have gotten brigher and brighter blue, and bluish areas first turned purple, and are now bright red.
The bigger problem coming is fiscal. There is flat-out no possible way to cover upcoming pensions and social spending, and that will hit the urban areas first, and hardest. All of these programs were predicated on the idea of a growing population paying in to cover those collecting benefits, and the ratio has shrunken dramatically due to longer lifespans and low reproduction rates, combined with automation reducing the number of workers needed in many industries.