It seems like the problem is that it's easier to make the normal internet produce the same behavior than to actually make a mesh network fast and reliable. If all your neighbors have normal internet with NAT-PMP supporting gateways then you can create a "community network" entirely in software.
Maybe the problem is they're doing it from the wrong angle. Don't create a mesh network independent of the internet, create a mesh network integrated with the internet. Then use whichever interface has the best performing route and use them to back each other. So if your wireless goes down you can still use Comcast and if your Comcast goes down then you can seamlessly use your neighbor's Verizon.
Maybe the problem is they're doing it from the wrong angle. Don't create a mesh network independent of the internet, create a mesh network integrated with the internet. Then use whichever interface has the best performing route and use them to back each other. So if your wireless goes down you can still use Comcast and if your Comcast goes down then you can seamlessly use your neighbor's Verizon.