I think what will eventually happen (I may very well be wrong) is that when there are several communities that are very similar on different instances (e.g. lemmy.one/c/aww and behaww/c/aww) one of them will eventually grow significantly bigger/more active than the other, and the other will be more or less abandoned, with its subs/mods moving to the bigger one.
That may not necessarily be a good thing, but over time I think thats what will start happening.