Ipv4 has lasted long due to technologies such as NAT and Rfc1918 ie private addressing, however it is unsustainable due to limited addressing space to go around over 4 billion users

It is not sustainable because of limited address space

Ipv4 has lasted on because, it is very simple to implement and it supports backward compatibility Further more, technologies like NAT and the RFC 1918 which is the private addressing system, CIDR and DHCP have helped extend the lifetime of Ipv4