I think the federated nature of Lemmy/kbin will be what keeps it going. It’s not an all or nothing approach but rather many pieces coming together to make something truly interesting. I’m sure there will be plenty of growing pains and drama, but the Lemmy-verse is a community built by the community.
While it ensures it continues in some form, it may also be the reason it never really reaches mainstream popularity. Which has it's pros and cons, but ultimately more cons than pros for content aggregators. The more people contributing the better (so long as they're not bots). Not only does it bring in more variety, but higher numbers mean cream will more easily float to the top and dregs will more easily be pushed down.