There are various reasons that the legacy IPv4 assignments are not returned. At the normal rate of IPv4 address assignment, circa 2011 when IANA ran out, reclaiming all those addresses would have delayed address space exhaustion by maybe 2 years at most.
All it would have cost was unpredictable, unbound man-years of work as each organization tracked down each of their publicly addressable machines, not all of which were recorded, and vacated into smaller subnets of their address space. I'm sure you know of the servers that sometimes disappear behind walls during renovations. And some of the older machines require the kernel to be recompiled and rebooted to change their IP address. Ah, those were the days.
We just have to face the fact that IPv4 was not designed to be used on a network the size of the Internet. NAT and CIDR have stretched it out amazingly, but we've stretched it out about as far as it will go. We need to switch to IPv6.
All it would have cost was unpredictable, unbound man-years of work as each organization tracked down each of their publicly addressable machines, not all of which were recorded, and vacated into smaller subnets of their address space. I'm sure you know of the servers that sometimes disappear behind walls during renovations. And some of the older machines require the kernel to be recompiled and rebooted to change their IP address. Ah, those were the days.
We just have to face the fact that IPv4 was not designed to be used on a network the size of the Internet. NAT and CIDR have stretched it out amazingly, but we've stretched it out about as far as it will go. We need to switch to IPv6.