Minimum connection fees, legislated enforced grid connect.

Or just let it go.

It'll still be a thing in cities because you don't have enough area to collect sunlight for tall buildings on the buildings themselves, but that's also where the costs are lowest because the density is highest. If technology makes it so you no longer need a power grid in rural or suburban areas, and the economics are no longer viable there, why should we pay a lot of money to continue to have one there?

The question is how long-term transmission costs will look. If cities won't be able to self-generate their own electricity, then the electricity has to come from somewhere. Currently we assume that it'll have to come from a centralized generation plant, but why?

Hypothetically, if the capex to install PV is low enough, and the transmission costs are low enough, then you could build a type of decentralized generator where suburban and rural generation feeds urban demand. There's early work being done on "smart grids" which enable this pattern, but I could imagine a future company that markets to a private homeowner, one who already had sufficient generation for their own needs, with the offer of upgrading their private PV in exchange for most of the generation profits.

That would require you to continue to maintain the grid in suburban and rural areas. And then install more panels there, so they can generate for the city as well as themselves. It seems like it would be a lot more cost effective to just put the additional panels (or whatever other generation method) in a few centralized locations outside the city and then only have to wire those specific locations up to the grid.