On a user-driven platform, not all users are created equal. Lurkers bring little to no value to the platform beyond clicks. There might be a huge engagement difference on a per user basis.
Moreover… I just want my niche communities to be active. We will never have Reddit’s archive of content, but we can get to a point where the Lemmy’s corpus of knowledge grows to at the same rate as Reddit’s. I don’t know how many users it’ll take to achieve that; 500k? 1m? 2m? 10m? No one knows that number, but to me that is the number to beat.