Great reply, much appreciated. I searched a bit for a number like ~70B and didn't find one. Perhaps 36 bits wouldn't have actually worked. I do think we'd have wasted more Class As, but that's where the "minor market mechanisms" would have happened—most of the class As did get sold and would in this universe too. Again, if total internet-connected devices is now 70B that wouldn't help, you'd still need NATs.

It's extremely hard to get an accurate count of connected devices, so we're all just estimating. There's lots of sources giving rough values for things like smartphones or IoT devices, there's a reasonably common estimate of 5.6 billion connected Internet users, but it's largely guesswork for connected devices per user.

It's improbable that I'm off by an order of magnitude: 7 billion is far too low (we have 7.5 billion smartphones in the world!) and 700 billion is far too high; how low an estimate could we make without being unreasonably optimistic? 40b seems quite low to me - 7.5b smartphones, 5.6b connected users, 20b IoT devices, and commercial use of IPs - but if we took that value we'd be sitting at saturation for 36 bits of address space (60% utilisation is pretty darn good!) and the next decade would kind of suck.