That was always the plan for "the future". That is get everyone to IPv6 and then get rid of IPv4. IPv4's days are numbered - but the number looks really big.
Why would we keep around a whole separate Internet? Dual stack was always only intended for the transition period.
It's very hard to get rid of old standards.
"The past is never dead. It's not even past"
Of course there's an incredibly long tail here, but in the big picture, "nobody except some people maintaining a few legacy systems ever need to learn to work with this protocol anymore" is practically the same thing.