You're asking to rely on a smaller company with fewer spectrum licenses, fewer towers, and therefore spottier coverage.

You might have more towers, or you might have all 5 companies on the same set of towers. Regardless, I think the spectrum is fully sold, so a challenger network isn't going to have spectrum to use, and if they get spectrum reallocated, existing towers that lose spectrum will be less effective.

Edit to add: If your towers are as congested as they seem, the carriers should be aware, and the problem is likely a lack of available tower sites; either because of geographical considerations, site owners don't want towers, or local regulators don't want towers. Additional networks won't really help with that either.

I think it is more likely that they'd all outsource tower construction to a small handful of companies and leave congestion roughly equivalent to the current state.

TBH, this happens a lot in large stable industries.

The solution to congested towers is to add more towers and reduce the power levels making the coverage of each smaller. In a crowded room you can increase the number of simultaneous conversations by having people stand closer to each other and reduce voice volume levels at the limit people are whispering into each other's ears. Similar idea works with wireless devices. devices.