Still, even when I joined in 2017, there was a lot more conversation going on in the NL sub. I remember adding to many discussions when I simply just registered to trade some furniture for my design ideas in NL. I feel like part of this is also because NH had a slow update feed, that Nintendo stopped support so early and what felt abruptly, and overall there's not a lot of content in NH besides being a design sim. So it is a tad bit superficial even if a QoL overhaul. I enjoyed it, but I wanted more out of it. And a lot of old fans shared that sentiment and moved on before the game even got the 2.0 update. I think this also aided in the decline.yeah this also happened like 4-5 years after new leaf was released, when I started being active on here in mid-late 2016 there weren't many active people at all. that would explain why the 2020 fair had a huge number of people participating in the 2020 fair, compared to the 2017 fair which didn't see many active participants and thus there are way less collectibles remaining from it (like the fresh, rad, sweet, and final boss feathers). and ofc there are a lot of people who aren't super active but do pop back on for raffles and events, as you suspected.
it is interesting to note, though, that people started leaving this place pretty quickly after New Horizon's release (about 2-3 years) compared to New Leaf (about 4-5 years). I remember the market for buying/selling in game stuff was still pretty strong in early 2017.
I just hope in late 2020 when the next AC comes out that Nintendo makes a definitive edition to what AC should be and borrow from all the elements that worked with all the previous entries including NH.