If you want an example of community consolidation between different servers, there is !unixporn@lemmy.ml and !unixporn@lemmy.world. Most of the activity if happening on lemmy.ml, and there was a backlash when the mods from the sub wanted to takeover the lemmy.world community. Both are still open, but someone who wants to post would see that the LW isn't as active as the .ml, and post to the latter.
About your point, there are two things at hand.
First, the technical possibility of it happening. It has been linked elsewhere, the Lemmy devs are not interested in this. Kbin has it, somehow, but the userbase is now on Lemmy, and I don't see it moving the Kbin/Mbin, except if they surpass Lemmy in features. Maybe sublinks will have this, we'll see.
But beyond the technical aspect, there is also the "political" aspect. People who don't like communities on LW are not going to enjoy being forced to have their content shared to LW communities too. People who avoid Lemmy.ml due to the political stance of the users are also not going to be happy to discuss with the people they are trying to avoid.
The point is that Lemmy has been around for some months now, people know each others, the other servers, and more or less where everyone stands. If people keep communities separated, there is a reason, and it's not going to be solved by technical measures.