I think you will be surprised on how much stuff will break in mysterious ways. And that it will create a nightmare to actually operate. But even if we pretend i

I think you will be surprised on how much stuff will break in mysterious ways. And that it will create a nightmare to actually operate. But even if we pretend it will just work there is the still the issue that IPv4-only hosts cannot talk to IPv4-NG hosts. There is no way around that. And having to upgrade the core is not really an issue (anymore). You are focusing on the wrong issue.