Stuff breaks. I fought this fight a few years back just to educate myself, and the mere presence of IPv6 on the network, DHCPv6 addresses being handed out, AAA

Stuff breaks. I fought this fight a few years back just to educate myself, and the mere presence of IPv6 on the network, DHCPv6 addresses being handed out, AAAA records being returned from the local caching DNS, etc... made all sorts of software loopy. One I remember in particular was that if you hit a default openssh configuration from the local (!) network, even on a link-local address, it would try to do a RDNS lookup and take 6 seconds or somesuch to time out. I remember a coworker telling me about a TV that would request and accept a DHCPv6 address and then fail hard getting to the internet. Wifi router firmware likewise messes things up, etc... It frankly just wasn't worth the hassle. Mobile networks that can control and enforce the full stack have been able to make it work. My guess is home/wifi environments will be IPv4/NAT until the end of time, frankly.