I don't think that it can be argued that IPv6 was complex and demanded more from the networking hardware.

On the contrary, the IPv6 headers have been explicitly simplified a lot in comparison with IPv4.

The increased complexity for the networking hardware has come only from the requirement of supporting both IPv4 and IPv6, instead of only one of them.

In my opinion, the most significant mistake of IPv6 has been that the IPv4 address space has not been considered a subset of the IPv6 address space.

Had this been done, it would have been relatively easy to mix arbitrarily in a network ancient IPv4 equipment that has never been updated with equipment implementing IPv6 and the gradual transition between the two protocols would have been very easy.