Lemmy and kbin are still very much in their infancy while Reddit has been around for nearly 20 years. There's definitely going to be growing pains since it's still so new and for right now at least most people that are going to join are people that are leaving Reddit so it's probably going to remain a fairly niche thing at least in the short term.
Secondly, I don't think they have to necessarily replace Reddit and they both could go on and do their own things. Honestly, even during my time on Reddit I found the smaller communities were better than large or default communities IMO. I just find that smaller communities have better engagement where ones that are too big your post or comment will end up getting buried.
Even if Lemmy or kbin don't overtake Reddit, I think there's still a place for them.