Well yeah if you made your router use 2002:1.2.3.4, your ISP advertised 2002:1.2:: on BGP, and the other ISPs agreed your ISP owns that, that would work. They didn't do that, and the spec didn't say to. They did 6to4 instead.

I understand the limitation that you can never put a 128-bit address in a 32-bit field, and one way or another two hosts and everything in between have to understand the new packet format. That didn't force them to make ipv6 its whole separate network from v4 where almost no state is shared with v4. Having separate DHCP6 vs DHCP4 was a choice, likewise with DNS, NAT, and even the routing tables. It makes the difference for service operators who would be fine adopting ipv6 but don't want it to be a big project.