Nothing against that either but some people can't do it due to hybrid deployments, providers that don't give you a /64, or providers that don't offer V6 at all.

Currently Amazon, Google, and Azure have no native IPv6 support.

Also many are allergic to the security implications. You have to be rigorous with ip6tables and making sure everything speaks SSL or another encrypted protocol using authentication in both directions. Many things do not support SSL or don't support bidirectional auth.

Personally I doubt overlay networks are going away. Most backplane software like databases, distributed caches and event servers, etc. offers literally no security because it's all built with the assumption that it will run on a secure backplane. I have personally railed against this for years but I've found that it's a total waste of breath.