The biggest problem is that it's likely against your terms of service to share your internet. If this actually becomes a thing, then that will be enforced, likely with extra charges or termination. There are also likely anti competitive clauses for areas when an ISP comes in, so unless there are multiple corporations running fiber to where ever you live (assuming it's not a city) then you likely have very limited options for a decentralized internet. Long range wifi is a thing and you can certainly have a mesh network that covers large areas, but it will be vulnerable without near 100% participation with long range antennas or a large amount of fiber or satellite trunks. This depends on population density and environmental factors like mountains.