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.
A huge userbase like reddit's is both a pro and a con. The big advantages include diversity of content (especially niche/hobby stuff), more content, and higher frequency of new content overall.
But it comes with some pretty big disadvantages, too. Moderation is difficult so they are happy to let a small number of "power mods" run everything. Subs that were fun & interesting in the past....after they hit the front page and become popular they go downhill quickly. Divisive USA-centric politics.
And of course the "asshole filter" effect: where the assholes drive away the non-assholes, so the concentration of assholes is always going up.
One of Reddit's biggest issues is mods controlling dozens of subreddits. So that means it feels a lot like corporate America where one company has their editorial say in everything.