I'd think content providers in general would be willing to provide bandwidth under a mesh network. They'd also be the ones most likely to pipe the mesh into the existing infrastructure. Who else besides the likes of Google / Netflix / Facebook would front heavier duty APs city-wide. Every ounce of traffic would have to be encrypted though. The main barriers for mesh networking when I look at it are:

First and foremost: Antennas and hardware in general. It's one thing to maintain a multi-channel 500mbps connection within 10-20 feet, let alone doing it with multiple devices simultaneously connected to other multiple devices. Even if you scale the speed down, the consumer grade hardware out there isn't targeting this use case.

Second: Protocols. If we had the hardware, the whole mesh has to be trustless while being affected as little as possible by connectivity and pathing overhead. Plus backwards compatible with existing infrastructure (which is less hard I guess).

Third: Gatekeepers. If you get them on board, creating the mesh as a natural extension of the internet becomes easy. Since they invariably won't be on board, it'll be that much harder.