If you join an instance that isn’t defederated with lemmy.world, you should have access to all of the same communities and comments that you’d see there. That’s kind of the entire idea.
For example, I’m viewing this post and these comments from sh.itjust.works. I had to go back to the top of this post to see that it originated from lenmy.ml
A decline seems natural. Of course there are many people who came to lemmy to check it out, and not all of them stuck with it. That is to be expected, no?