Because that would be starting from scratch the migration with all the warts and less of the benefits. About the only part that would be easier is that more pe

Because that would be starting from scratch the migration with all the warts and less of the benefits. About the only part that would be easier is that more people use less broken APIs compared to BSD Sockets, with getaddrinfo and similar solutions ported from XTI/Plan9 finally becoming the norm. So no more "you need to duplicate code to add support for new IP version". But that's still worse than migrating to v6, which has widespread hardware support in switching equipment, considerable preference in mobile networks and not only (TL;Dr it's cheaper to use v6 to carry v4 traffic using 464 stateless translation rather than other forms of CGNAT), and some IOT systems (industrial, not home stuff) are v6-only if they have IP connectivity on the embedded side at all.