How many base stations can you reasonably fit on a tower, how many redundant networks (even without redundant towers) can the market support? Let's say... 3 or 4?

What’s a “fair price”?

Like I wrote: covering the cost of establishing and maintaining the tower.

It's essentially a matter of declaring that the towers themselves, and the infrastructure that connect and power them, are not to be a profit maker for anyone. Can be made a condition during spectrum allocation, for example.

There's still plenty of other areas for providers to distinguish themselves.

And there's still an incentive to increase coverage. The only coverage increase incentive lost is the incentive of becoming a local monopoly. Good riddance, I say.

No private business is going to invest just to have its upside limited.

It's not its only upside.

They're already required to make sure the tower doesn't fall in someone's head. That cuts into profit. But they still do it.