v6 adoption is often an all or nothing, because if you run both stacks, you have to ensure they are consistent. While you can reasonably do it on your home LAN, doing it across an entire infrastructure is the worst.
Now you have to make sure all your subnets, routing, VLANs, firewall rules, etc work exactly the same in two protocols that have very little in common.
It is the equivalent of shipping two programs in different languages and maintaining exact feature parity between both at all times.
v4 was built around the idea of multiple free standing networks linked by gateways. v6 was built around the idea of a universal network.
I dont care about what your LAN adress space look like when I'm in my LAN, because we are not in the same v4 network. I am sovereign in my network.
With v6, everyone is effectively in the same network. I have to ask my ISP for a prefix that he will rent me for money even for my LAN. If I want some freedom from said ISP prefix, I am mercifully granted the honor of managing ULA/NAT66 (granted I paid for a fancy router).
Also if I want any kind of privacy, I will have to manage privacy extensions and the great invention of having to use automatically generated, dynamically routed, essentially multiple random IPs per interface. How lucky am I to use such a great new technology.
Seriously v6 was created by nerds in a lab with no practical experience of what people wanted.
