There's already a lot of traffic on Lemmy. I'm constantly surprised to see posts with 400+ responses. I think it's already hit critical mass (Enough activity to keep people here).
Mainstream is also what killed Reddit, better to have a "big enough to be good" community. I almost appreciate that the barrier of entry is slightly higher.
I don't think being mainstream can kill this in the same way due to the nature of how instances are hosted and their level of customizability. Even if it gets hyper popular and there are communities that rival the biggest subreddits, there will always be smaller niche communities that fit the bill you're looking for. If they don't exist you can simply host your own