I don’t really think that Lemmy or Mastodon will really replace their counterparts. At least not for now. As many have already said, the federation system is too complex for many non-technical people. It would take something like a de facto standard app, that abstracts everything federation related away and make it feel like another centralised solution.

Another point for me is the searchability of federated systems. Say you are searching for a technical problem right now, google will surely bring you to a related subreddit in just seconds. I have yet to see a Lemmy related search result.