Republicans are not going to win cities or get anywhere close for a long, long time, but the margins they're getting matter a whole lot. If Democrats need to win Detroit by 80+ points to win Michigan, then Republicans even getting 15% in it, for example, would hurt Dems a great deal. Perhaps the suburban trends could make it a moot point, but relying on getting basically every vote in many urban areas provides some shaky ground in a lot of Midwestern states. They can only go down, and as long as Democrats have dominated urban POC, that seems to be slipping. If Black voters start voting 80% instead of 90-95%, that would be it for Democrats in the Wisconsin, Michigan, Ohio, Pennsylvania, etc.
At that point, Illinois and New Jersey would likely come into play, with New York also getting closer. If anything, maybe improving with the urban black vote is how Republicans can stop the bleeding in Georgia, although the dynamics there are different from the North.
