Could you be more specific? What about this proposed "ideal" scheme is infeasible (specifically the engineering limitations you mentioned)?

As a quick example, I’ve been doing a bit of work with land-mobile radios lately (LMR). We rent a very small slice of spectrum from the government (12.5kHz wide, IIRC), and all of the radios use that frequency. They work anywhere (within the range of our license anyway). They run independent of a repeater or any kind of fixed base station.

If I’m out in the middle of nowhere with my team and we’re using these radios because there’s no cell coverage, a) how would a given radio “bid” on a piece of spectrum, and b) how would the other units in the talk group know to listen to a particular frequency for my message?

That's a neat problem. You may hear N other stations, but they may not hear each other at all. Its very difficult to know whom to tell about coordination information, or even how to calculate it. Solve that, even heuristically, and make a $B

I agree! It’s a very interesting distributed system problem and honestly, I might end up doing some sketching tonight.

Right now it’s easy to know whom to tell about the coordination though: the Canadian government :).