Sure, you can connect the towers to the carrier however you want, and this is why certain carriers can sometimes hack in better quality protocols within-network (see the discussion on voLTE further down the page). Carrier-to-carrier connections, however, still have to use POTS/PSTN (since, if you send a voice call to another carrier's network, you can't a priori know it's a smartphone vs. a landline vs. something else right now, and PSTN is the only standard they all agree on). I agree, it's dumb.

Also see https://en.wikipedia.org/wiki/Public_switched_telephone_netw... regarding the 64kbit/s limitation.