IPv6 has other differences than the address length. A theme for IPv6 was to make it even easier for network equipment to process to make it faster; which aligns with "dumb fast scalable core; smart expensive edge" Internet philosophy - this is in contrast to "smart hard-to-expand core; dumb cheap edge" phone system philosophy

Compare the packets. https://www.networkacademy.io/ccna/ipv6/ipv4-vs-ipv6

Did the authors of the standard just assume that migration and adoption would be easier than they've turned out to be?

No, but IPv4 and IPv6 can be used together (they're separate protocols) so there was no need for that.