We are talking about software defined radio though. How do these facts apply when there are no bands anymore? When collisions are avoided probabilistically instead of by partitioning?
No change. Just because your radio is "software defined," doesn't change physics and signal processing math. There are 'best' ways to use a given piece of spectrum, and while a SDR can make your hardware more flexible, it can't change that the most efficient way to use spectrum is usually to have a dense set of subcarriers modulating amplitude and phase.
By avoiding a strict multiple-access protocol on a band, everything just gets worse; probabilistic back off is inefficient vs hard-scheduling.
Every wireless system I've worked on (satellite radio, WiFi, LTE) has been very spectrum constrained, so I dont really believe in this future where spectrum is cheap.
In some sense,mesh WiFi doesn't seem spectrum constrained(considering home use) - just install enough mesh AP's.
Won't work unless the AP's scrupulously adhere to protocol to avoid interference. In practice the 2.4GHz interference has became tragical. In European high density housing, you are likely to have precipitous speed drop moving only across the room from the AP or behind one wall.