Sure, 6-monthly users is increasing (and plateauing) and people sure are posting more comments, but those graphs do not paint a good picture and do not suggest positive user growth.

Yep, this graph basically shows that growth hasn't stopped, it was just overtaken by the drop in Lemmy users. I will return to it a bit later.

Every time someone says “Good, less users is a good thing”, they’re saying nothing needs to change because that’s what they want

Only if it's taken outside of context. Okay, I admit I shouldn't claim "No one said that", but in many cases people aren't celebrating the decrease of Lemmy users. For example, OP clearly stated:

It is probably due to a number of people stopping using their alts after some instance hopping. Also a few people who came to see how it was, and weren’t attracted enough to become regular visitors.

From my perspective, this decline is a consequance of a rapid growth during last months: people were promised with a new reddit, but they got lemmy, with its quirks and issues. Of course, some people weren't satisfied with it - and when protests on reddit came to an end, they could finally abandon lemmy for the platform they were actually interested in.

That's why I pointed out on the fact user growth never stopped - Lemmy's still attracting new users, just people who weren't interested in lemmy in the first place decided to leave.