[welterde]

IPv6 is pretty much IPv4 with more bits - at least compared to the alternative of the time that was CLNP/DECNet (which had 20byte addresses and is quite different).

And the very slow transition hasn't really anything to do with the standard itself, but with the transition technologies (NAT64 like you mentioned). It would have happened with any of the proposed alternatives. Hindsight and all.. NAT itself wasn't even a thing yet (not an RFC anyway) when IPv6 was being developed. Flag days had worked in the past too, so why not again?