Should Democrats be concerned about maintaining their margins in urban centers?

[DaleCooper]

They shouldn't take it for granted, that's for sure, but they shouldn't be unreasonable and expect the "emerging Republican minority coalition" or whatever Atlas thinks is going to happen.

[Skill and Chance]

Illinois is a significant medium/long run concern.  At this point, losing some ground in CA and NY would be net good for Democrats in terms of narrowing the Senate/EC gap and getting their national messaging more in touch with the median congressional/EC voter.

In terms of giant cities that aren't tech-oriented, there's plenty of room for error in L.A. and NYC, but Chicago could eventually give out and if Dems can't move the needle any further in Houston, it basically closes the door on Blue Texas.  But again, it's more viable to be the party of medium-size states in the long run. 

[Bootes Void]

NY and Illinois will not be in play anytime soon