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.
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.