I am not sure why you are downvoted, since you do make some valid points. I myself am not sure whether the very fundamental changes that IPv6 brings did not hamper its adoption than if it was just extended IPv4.

From a network administration perspective, sure, "you need to replace every piece of equipment". But from a software modification perspective (thinking more about the software on network infrastructure equipment than endpoints like applications), you have two very different stacks.

On the other hand, if there was ever a time to make (assumedly so) highly desirable compatibility-breaking changes, that was it.