As an interesting illustration of this idea, RuneScape (a popular freemium MMORPG) has done just this: they split up into the modern game and "Old School" (OSRS) which is based on the game in 2007. Some people heavily prefer one or the other, and by now the communities feel very distinct.

I don't know exactly how well it's working out for them. Now they have two games to support and promote, with about the same number of users. This number might (likely?) be higher than if they had stuck to providing a single game, but I'm not sure. It's hard to tell.

There seems to be some business risk to doing this. Apart from possibly cannibalizing your main product, it also requires two teams to support and run the two games. If you actually want the experience on the old-style server to be at all stable or compelling, you'll have to put some time and energy into them—it's more than just running the server.

Honestly, I have no idea if what RuneScape did was the right move, but it's a really interesting case study.