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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.