If some people can still get online via cellular or satellite, then surely most people can still get online?
Not necessarily true, actually one could list dozens of scenarios in which your argument doesn't hold.
There is pretty much zero incentive for people to set one up or maintain it for some very hypothetical situation in which not trading bitcoin would be the least of your concerns.
It is not just about trading Bitcoin, in the bigger picture, it is about preserving tcp/ip connectivity exploiting its essential inherent decentralized nature and resistance against single points of failure. As of your lack of incentive argument, I suppose it is an open issue that needs further innovations and more elegant ideas.