I don't feel strongly about it, but my reading of the complaint is that the implication is it should've been something like an address in 240.0.0.0/4 plus more bits. Then to IPv4 it looks like a reserved for future use address (and we declare we're done with IPv4 now, there will be no (other) future use) but to (this hypothetical variant of) IPv6 it's a longer address.

I think what annoys people is everything else that changed with it, if DHCP (I know), subnets, NAT if you wanted it, etc. was all just the same, if the model was the same the IPs were just longer now, that they wouldn't really complain.

I'm not even sure it would be necessary to put it in unaddressable v4 block, since it would be too long and a different version anyway. Obviously people have thought about this a lot more than me so know a lot more about it, I'm not naïve about that.