... and they were right.

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.

I genuinely don’t understand this. The concepts are nearly identical between the two.