I don't think whether Gen 6 has been successful or not has much to do with it, both XY and ORAS have been pretty popular with the general fanbase just as Pokemon always has been.
I've said it before but I mean there's a ton of social media outlets around that people use these days so forums kinda "compete" with that. Forums are old-school in a way and people are more impatient so they don't want to wait more than a minute for anything (citing people joining IRC channels to say "hi" and leave like 20 seconds later if nobody immediately responds). I can't see a lot of forums being as active as they were 4 or 5 years ago but yeah some of that emptiness here can be attributed to the fact that there's no main series game to talk about.
This. I know lots of people in real life who use serebii, none of them use the forums.
A lot of this kind of discussion now happens in other places. Reddit isn't so different from a forum, but I am really sick of people trying to treat facebook like one. And most other social media outlets aren't the same either. (though I do have a friend who is real active on some of the meme/ net subculture parts of facebook and it is kind of insteresting. though I am talking about really weird trippy stuff, not nerd /4chan style stuff)
That combined with effective modding here pretty much killed off the trolls that made all the threads and such here.
This isn't entirely inaccurate either. Not so much with literal trolls (though some of them where really entertaining and not all that harmful), but with the community around misc. in general. (I understand a ton of people who weren't trolls got banned for alts for some reason, and a lot of others just left)
You'd have a bunch of people posting on a pokemon forums who acted like they where too cool for pokemon. I am sure a lot of the serious pokemon fans who post here would scoff at the idea now, but it made the forums more interesting. Just because it's a pokemon forum doesn't mean people want to talk about pokemon 24/7. You cycle through all the obvious pokemon topics pretty quick, leaving there not much to talk about on a regular basis. Off topic discussion keeps people around longer. Provided it is decent off topic discussion that involves interaction and not "what's your favorite X threads or the like."
Not blaming the mods or the mod policies here mind you, just providing somewhat of an explanation. (I am sure the mods get tired of pokemon discussion too) I actually know of other forums around this scale that aren't so dead, even with stricter moderation, because they have a stronger community.
I do miss the whole trend of bragging about how cool you are in general chat though.
*I think this applies to stuff like other anime and other video game sections as well really.