So either the new octet is in the least-significant place in an ipv40 address, in which case it does a terrible job of alleviating the IP shortage (everyone who already has IP blocks just gets 256x as much as them)

Or, it’s in the most-significant place, meaning every new ipv40 IP is in a block that will be a black hole to any old routers, or they just forward it to the (wrong) address that you get from dropping the first octet.

Not to mention it’s still not software-compatible (it doesn’t fit in 32 bits, all system calls would have to change, etc.)

That all seems significantly worse than IPv6 which already works just fine today.