If 2 people refuse to change their software, then they will continue to communicate with each other as peers and will continue to operate the old protocol. They will continue to call their system "bitcoin", and their system will still be the original "bitcoin". Any other system will be an altcoin.
I don't really agree. If everyone who identifies as a Bitcoin user switches to a new protocol, except for 2 people using the original protocol, then the majority will be using "Bitcoin", and the minority will be using "an outdated, unsupported past version of Bitcoin".
They can shout "But we're using the real, original Bitcoin!" as much as they want - but for better or worse, it wouldn't change a thing. History written by the winners and all that.
Here's another way to look at it. If the creators of dogecoin had decided to name their system "bitcoin" (and leave everything else about dogecoin exactly the same), there would obviously be a lot of confusion as people tried to talk about exchanging legacy "bitcoins" for "new bitcoins" or vice versa. However, the fact that the new "bitcoin" (dogecoin) (an inflationary altcoin based on the legacy bitcoin protocol) is popular and has many users does not mean that the original "bitcoin" system has become inflationary.
It matters whether the users of the new protocol are new people, or people who originally used the old protocol but switched to the new protocol.
If the former then yes, it's just two coexisting, similarly named groups. If the latter then no, I would say the Bitcoin system really did evolve into something new.
And at this point we are discussing semantics and can't proceed with constructive debate unless/until the OP defines exactly what he means by "more than 21 million Bitcoins". We need the OP to define exactly (at a technical level) what "a bitcoin" is meant to be in this discussion so that we can determine if it is possible to have more than 21 million of them.