Was there any tendency towards centralization of subreddits?

That seems like a different dynamic to me. Subreddits have a divergent tendency because people have different interests, tastes, etc.

But new communities can all be created all on the same big Lemmy instance, each on its own instance, or anything in between.

To take the extreme example of one community per instance—I don’t think we’ll see that because spinning up and maintaining a new instance would be an incredibly high cost (time, if not money) just for someone to start a new community.

Even in an in-between state where there are dozens or hundreds of non-trivial instances, someone deciding to start a community would be incentivized to do so in the most popular instance (or one of the most popular ones) because the community would be visible to more people more quickly (since non-local instances have to discover it first).

But to your point, this depends on how much of an advantage it is for a new community to be instance-local. In my (very limited) experience with Lemmy so far, there’s a definite difference in ease of finding/subscribing locally vs on another instance.

Maybe this can be addressed in time. And that’s kind of the reason for my post—I think it’s worth thinking about what dynamics might bias things toward or against centralization, and trying to keep the balance tilted toward decentralization.