The authors of IPv6 intended to replace v4 with it, according to https://web.archive.org/web/20190129005124/http://www.ipv6.org.nz/ipv6-faqs/ https://web.archiv

The authors of IPv6 intended to replace v4 with it, according to https://web.archive.org/web/20190129005124/http://www.ipv6.org.nz/ipv6-faqs/ https://web.archive.org/web/20190129005124/http://www.ipv6.o... Presumably the next several billion devices want to talk to the current ones. There will always be old devices left behind on old protocols, same as how old versions of SSL get rejected everywhere, and they can deal with it via compatibility layers like 4-to-6 NAT.