That’s still a limitation on bandwidth, not data volume. It’s still the bandwidth that costs money, not the volume.

Not really. OFDMA and other modulation mechanisms for doing dense wireless connectivity do have limitations on number of active connections based on frequency (not necessarily data bandwidth) available. Someone communicating constantly will eat up way more slots than their neighbors.
https://www.5gtechnologyworld.com/the-basics-of-5gs-modulation-ofdm/
Wireless is a shared resource that cannot be guarded. This is not the case with cables... Where that bandwidth limitation is never encroached upon (short of the North American Fiber-Seeking Backhoe... Shown here:)

In short, someone taking less slots means that service for everyone is better. A cap can keep those slots open as people would be incentivized to use it less.

The alternative is that they install more wireless transmitters but dial the power down so there's more cells. Except this will have alternative problems in penetrating into buildings and such. So that's not really an answer either. And with way more hand-offs you'll run into more problems using your cellphone anyway.