People couldn't care less if it's centralized or not. People come for the community, not the tech behind it. Also people are lazy, they will use the easiest thing that comes up. Why should one go to another instance, if the one they are right now works great?
I am not saying that this system is bad. I am just saying that people will always take the easiest option there is.
Probably because the current system works for them, and changing it would cost political capita that could be used elsewhere.
I think the federation system is quite a barricade for some people. I recently changed over to Lemmy from Reddit, and it was quite challenging to first understand how it worked. Also I am not sure that I even now understand how this works fully.
Also the smaller community on Lemmy might be a turn-off for some people.
How I see it is that every decentralized system with people is going to form some sort of centralization unless you actively fight against it.
In Lemmy's case, new people will check what are the biggest communities and go there, and since there are more people there, it attract even more people. More people, more communities in that instance.