The reality is that forums in general aren't as popular as they were during melee/brawl era, melee fans in general are older that the other smash fanbases (exception smash 64), and you need to consider that melee top players don't really use forums or discords (aside of ppmd who still post here), because twitter and twitch are far easier platforms to get views that get money
This is not true, you can take a look to
www.comicvine.com, currently, a CO forum which was approximately created in 2006, 7 years after Smashboards was created, and their activity is actually insane.
It's not only about top players leading a discussion but the regular dedicated players too with aspirations to become professional players, that's the real core in order to keep a constant qualitative growth, the successors are important as the top players and places like this are key, one thing is that the staff didn't know how to deal with it, everything froze due the lack of care, persistence and recognition toward a more valuable community.
It's demonstrable, how? Look at Brawl how it died and then Smash4, now look to the Melee community, the ideologies behind those communities are insanely different from each other, they are not equal in any sense.
One side inherits and needs successors and the other side recruits units through hype and fashion, because of the product, the Melee community is practically homeless, because they can't find a house in places like in Reddit, but Reddit is a perfect home for the Smash4 and Smash Ultimate communities.
Despite the Melee community is homeless, it's a more solid community than the Smash4 community, how you can explain Melee's survival after all?