I wouldn't worry about Lemmy having as many users as reddit in the short term. Success is not just a measure of userbase. A system just needs a critical mass, a minimum number of users, to be self-perpetuating. For a reddit post that has 10k comments, most normal people only read a few dozen comments anyways. You could have half the comments on that post, and frankly the quality might go up, not down. (That said, there are many communities below that minimum critical mass at the moment.)
Lemmy is now a real alternative. When reddit imploded Lemmy wasn't fully set up to take advantage of the exodus, so a lot of users came over to the fediverse and gave up right away. There were no phone apps, the user interface was rudimentary, and communities weren't yet alive. Next time reddit screws up in a high profile way, and they will screw up, the fediverse will be ready.
Lemmy has way more potential than reddit. Reddit's leadership has always been incompetent and slow at fixing problems. The fediverse has been very responsive to user feedback in comparison.
"You could have half the comments on that post, and frankly the quality might go up, not down."
This is probably my favorite part of Lemmy. The comment section feels more meaningful, and not a landfill of garbage posts. Additionally, if I make a comment, there is a higher chance that it will be read and responded to, so it feels like I am actually engaging with a community, and not just chucking my thoughts into space and hoping they land on a planet.
I think the biggest value Reddit had to humanity was its original content. The kind of stuff that has people putting "reddit" in their Google searches for myriad topics.
As such, I'm not hung up on the numbers. If one really looked at it, that content generation is such a small fraction of what activity goes on over there. I'll take quality over quantity here.