Definitely there are fear of unknowns to deal with. And generally some business won't want to pay the switching costs over something perceived to be working.

Definitely there are fear of unknowns to deal with. And generally some business won't want to pay the switching costs over something perceived to be working. IPv6 is simpler in a lot of ways than ipv4 - fewer headers/extensions, no support for fragmentation. What makes it more complicated? What makes the failure modes more obscure? Is it just that dual stack is more complex to operate?