> Wouldn't this proposal not require isps to do anything? They already assign every user a unique ipv4 address.

The reason there's an IPv4 address shortage is because ISPs assign every user a unique IPv4 address. In this alternative timeline, ISPs would have to give users less-than-an-IPv4 address, which probably means a single IPv4x address if we're being realistic and assuming that ISPs are taking the path of least resistance.

There aren’t enough IPv4 addresses to give everyone one. That is why ISPs use CGNAT to hide multiple customers behind one IP address.

Something that just uses IPv4 won’t work without making the extra layer visible. That may not have been apparent then but it is now.