> 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.