going by the Federation's data, there remains a large gap—but beehaw is definitely one of the most active lemmy instances at this point and is definitely getting some cross-pollination from the main instance. (we're also the top-recommended instance on join-lemmy, i think)
I don't really think Lemmy.ml will have very good retention of new users. Because:
New users will mostly come from reddit or mastodon.
Most users of reddit and mastodon willing to leave are anti corporate, anti authoritarian, anti-war, pro human rights.
A lot of posts that you see on Lemmy (either from Lemmy.ml or lemmygrad) are pro-russia, pro china or anti-western to the point of being really annoying. Its not fun to talk to people without a hint of nuance.
So it is not surprising that new users will get turned off by this and leave (or move to beehaw).
I confess to trying and then leaving lemmy.ml in a huff because I allowed myself to get dragged into an argument with some asshole. I came back (to Beehaw) when I heard about a potential reddit exodus, and thought I'd give it another chance.