I say they do this now, and in 5-7 years if Lemmy is lucky enough to be a strong reddit competitor, then they could do any number of things to solve the centralization problem. They could easily stop accepting signups to lemmy.ml and put a new default in its place to help spread the load.
Right now, lots of reddit users are bouncing here to check it out, and bouncing back due to the complexity of signup. That is wasted potential to onboard. Fear of centralization is not Lemmy's primary issue at the moment. Lack of network effect is.
I agree that they should ask the community. But that's kinda what I'm hoping to do for them with a post like this. If this post gains traction I hope they take notice.