Ok I'm just thinking out loud here...

Part of what makes (made) reddit great is that there are so many niche communities. That you scroll through reddit and find something interesting and new and can just jump in and discuss it.
THAT I think we can all agree would be great to grow in lemmy.

There is no question that lemmy IS the same social media "mode" as reddit, a link aggregator where users can democratically sort news and articles and topics and discuss things. Lemmy is a clone of reddit, this can't be disputed. The question is how to make it better than reddit and avoid the pitfalls. Right now moderation and admins are a bit problematic like e.g. the recent vegan clusterfuck.

And yeah for certain subs like vegan you don't need downvotes since there wouldn't be too many "controversial" topics. I suspect part of what is lacking on reddit is that there isn't enough "tagging" on reddit. Like having ways for users to mark a post "funny/silly" or "unconstructive" or "misinformation" or NSFW, NSFL or hatespeech. Besides upvoting. Maybe I'm wrong with this.

But I think over time reddit degraded because the system didn't support protection against malicious actors. For example tons of meta jokes. Every serious topic has a joke comment on top. You can't filter them out. That might be a great use of AI like chatGPT for this where the AI learns from the tags and then allows to filter out joke comments. Not censor but allows you to filter out things that can be fun and crass but are not good for long term community. Basically to help moderators.

Of course the current problem is that users themselves have become more and more "post-truth", not just the fascists but the "leftists", liberals and centrists too and shout down any dissident opinion because they assume it's bad faith. Nobody wants to engage any more because they've been burned. Maybe that can be reversed if lemmy has the right tools and the community is moderated well enough to "heal". But again, the admins and moderators are currently the problem on lemmy. Lots of power-tripping and radicalism (and I don't just mean the socialists lol).

And voting with your feet is also a problem because it leads to fracturing like mentioned in OP and that does have a negative effect. Like the europe@feddit.de community lost 75% of it's users with the move to feddit.org. I assume similar things will happen to the vegan community now that you moved and people searching for vegan will be confused. Better technical tools are required.

So I think it's not just about surpassing reddit in numbers, even though at least some growth is very curcial for niches to thrive, but to grow the usability of the software.

Anyway, just rambling / thinking out loud 🙂