Now try that on an ipv4-only host trying to talk to an IPv6-only address. You can’t, hence, 2 internets.
(Yes, I understand there is no other way for a larger address space to exist that doesn’t have this problem. We are in violent agreement here. But that doesn’t mean there aren’t 2 internets. Let’s call a spade a spade.)
I can do it with a moral equivalent of a port forward.
This situation was inevitable, but it's not like they just gave up and said "welp, guess they're separate then". We've developed basically every single method of communicating between v4 and v6 that are possible to develop. There's two address spaces, but they're linked into one Internet.
(Or really, with NAT in the picture, it's millions of overlapping address spaces, but we still consider that to be one Internet.)
> If everything in the IPv6 protocol suite was literally identical to IPv4, but just with some extra bytes in every address, because they are not mutually compatible.
This part is true though. v6 is backwards compatible in rather a lot of ways, but v4 isn't forwards compatible, and it's important to note that this comes entirely from the larger address size -- no amount of making v6 more identical to v4 than it already is would make v4 any more compatible with it, unless you undermined the entire point and made the addresses 32 bits.