At the time when it was designed, IPv6 was well designed, much better than IPv4, which was normal after all the experience accumulated while using IPv4 for many years.
The designers of IPv6 have made only one mistake, but it was a huge mistake. The IPv4 address space should have been included in the IPv6 space, allowing transparent intercommunication between any IP addresses, regardless whether they were old IPv4 addresses or new IPv6 addresses.
This is the mistake that has made the transition to IPv6 so slow.
How would you have implemented it that is different from the NAT64 that actually exists, including shoving all IPv4 addresses into 64:ff9b::/96?
See IPv4-mapped ("IPv4-compatible") IPv6 addresses from RFC 1884 § 2.4.4 (from 1995) and follow-on RFCs:
* https://datatracker.ietf.org/doc/html/rfc1884
* https://en.wikipedia.org/wiki/IPv6#IPv4-mapped_IPv6_addresse...
Header processing and alignment were an issue in the 90s when routers repurposed generic components. Now we have modern custom ASICs that can handle IPv4 inside of a GRE tunnel on a VLAN over MPLS at line rate. I have switches in my house that do 780 Gbps.