Furthermore, even if they would manage to return the blocks to the pool in a couple of years, it would both be too late and too little and the demand for address space far outpaces the supply that ipv4 can offer.
This. We got 7 billion people - probably closer to 10 before it peaks, and as a minimum I should have one IP address at home, at work and for my cell phone. So 3*10 billion is 30 billion, IPv4 can offer 4 billion. And that's not counting every other odd thing I might want, like remote-controlled alarm/heating/whatever at my cabin or my car, servers of various kind and maybe IoT will become good for something.
Of course they probably could have just done it much, much simpler by making a dotted quad a dotted quint:
For compatibility each host under 18.104.22.168.x is granted 256 ports IPv4 ports mapped from x*256 to (x+1)*256-1 to a designated "IPv4 compatibility ports" like say the last ports from 65279 to 65535. So 22.214.171.124.1 can either be fully addressed by quint-capable equipment or 126.96.36.199:256-511 that'll be mapped to 188.8.131.52.1:65279-65535. And 184.108.40.206.2 will have 220.127.116.11:512-767 mapped to 18.104.22.168.2:65279-65535 and so on. You could use the same technique to provide a virtual IPv4 interface for legacy software, it thinks it is listening to 22.214.171.124:256 but it's really listening at 126.96.36.199.1:65279 - and any application it tells to connect to 188.8.131.52:256 would work.
That would have led to a gradual 256-times expansion of the address space without any hard switch-offs. But instead they decided to solve everything and now 19 years after the IPv6 standard we're still only barely in motion.