To be honest, I guess it's a mix of CoH3 not being interesting and the community here aging. I guess the majority here was in their teens to twenties when CoH2 released. Back then you just had time to fuck around half the day. Add ten years, you're working a 40h week, have responsibilities, family to take care of. Obviously activity here and gaming time will go down, especially if the 'new CoH' just doesn't live up to the expectations.
This forum has lived off of balance discussions and other platforms don't even come close. Reddit threads are short lived by design and discord is a mess if you jam a couple of hundred people into the same server. This forum was decently active up to the very last patch for CoH2 and has outlived many other traditional forums. The fact that this thread got quite some responses within half a day shows that people are still lurking.
But take CoH2 not being supported (so balance discussions being pointless), an maturing community, Relic losing trust and CoH3 being uninteresting together, it shouldn't surprise anyone that there is relatively little in depth discussion anymore.
Very well put, and a good addition to what Aerafield said. Even though the playernumbers a dwindling, there are new people to the coh-franchise because of coh3 and its very likely they are younger and thus prefer to use mediums such as reddit & discord.
That said, you cant have a meaningful discussion on those platforms because the hivemind will forget about it within hours as threads either dont exist or fissle out due to how the platform functions. On top of that reddit is just a mess with people not understanding what the up and downvotes are for.
Resulting in: Funny = upvoted to heaven, a well thoughtout comment will a medium amount of text = some upvotes, comments that are simply against the grain = downvote to hell.