Replacing implies taking something away, i.e. that the old item is no longer available and forcibly so.
That's not what happened though: what happened is that most Gnome and GTK developers discussed and designed - in public and for years, I may add - and chose to change what they are offering for free to the public. All of Gnome 2 and GTK2 code is still available, they just chose to move on and not work on that any more.
What is different in your hypothetical case? Who exactly would have taken over Gnome 2 for maintenance and development, in face of all the changes going on in the Linux infrastructure and needed hardware support? Who would have ported it all to gsettings and GTK3? Because I certainly don't see all that overflowing interest right now, and nobody kept anyone from doing so: Mate might have stricken the hearts of several Gnome 2 users, but is clearly a niche project and I have doubts on its sustainability.
If most of the knowledgeable people stopped maintaining Gnome 2 and moved en masse to Sparklynewname DE and GTK3+, with only a handful of new maintainers to keep the old code base in shape, I very much doubt that any distro worth its salt would have chosen - after an initial transition - to keep distributing a barely maintained Gnome over the much more actively developed Sparklynewname. Same situation as today, different names: it's not about the containers, it's about the content and the real people working on it.
All that said ( a rose by any other name ... ) I actually think that the Gnome devs had all the rights in the world to call their new project Gnome 3. Their work, their prerogative.