It's not that interest in gaming has gone down. On the contrary, gaming has never been more popular.
It's that people's attention spans have gone down. Reddit and Discord are the new hubs for community, not forums. Forums are for longer posts and discussions where everyone's word has equal weight and display without any filtering, requiring more time and attention to participate.
Reddit filters content by votes and recency, so less popular content gets buried, and thus users can access the popular stuff more conveniently. Comments can be posted without needing to reply or pay attention to anyone except the OP. Discord is just a chatroom where messages are generally short and brief, and they just come in a stream that allows people to jump in at any point and say anything. Cohesive conversations aren't easy in a crowded server.
In both cases, you can't make posts like I'm doing right now. On Reddit, it would get buried beneath the older, upvoted posts, and eventually the thread would be buried anyways. On Discord, people would just look over my post because it's long, and go back to simple chatting.
Also, forums aren't very conducive for phones. Discord and Reddit are. I know I wouldn't want to be typing all this on a small touchscreen.
A few years ago I used to be on the Metroid Database forums a lot. It wasn't very active, but a handful of users and myself managed to have some fun discussions and interact with each other. Then when they renovated the site, they shut the forums down, and there's still no ETA on when they'll fix it. But, they started a Discord server, and seem to consider that a replacement for the forums right now, when it's not the same thing. The interactions aren't what they used to be. I realized how little the people who run the site cared for the forums when they didn't even have it up before E3, when we all assumed we'd see Metroid Prime 4. Luckily for them, we didn't get anything, but I wouldn't be surprised if they just shut them down wholesale and stick with Discord, since it's free and requires less maintenance.
Compared to many forums, such as MDb, this site is much more active. And while I only joined a couple months ago, I can assume the lull in activity since Brawl to be due to two major factors:
1. Smash 4 came out during a time when Nintendo was the least popular they've ever been, with the Wii U living in the shadows of the PS4 and Xbox One, and in the middle of the years-long period where they were marketing to casual audiences, as opposed to the hardcore fans that would be on forums. Smash 4 wasn't as popular as Brawl, thus whatever activity it brought was proportionate. Brawl sold almost 14 million copies, Smash Wii U barely sold over 5 million.
2. It's been a while since Smash 4, and Ultimate is generating unprecedented hype, but it's not out yet. Right now, we're all just discussing our excitement, and only getting into concrete discussions on the rare occasion we get new information. We've only had two Directs and some gameplay here and there to sink our teeth into, and nothing else. Once the game is out, we can have more tangible discussions, and this place will be bustling. I expect Ultimate will sell over 10 million units in its lifetime, so there will probably be even more activity than there was for Smash 4, provided people opt for forums instead of Discord and Reddit.
What's ironic is that the "too big" meme was taken out of context. The full extent of what Sakurai was saying was "This character is a large boss in the games, able to pick up Samus and toss her around and do huge amounts of damage, so how do we size the character down and retain that essence?" When Sakurai said "It wouldn't be Ridley anymore," he means in the sense that if Ridley was just a reskinned Charizard, it would be Ridley about as much as Dark Samus is Dark Samus as an Echo Fighter. That is to say, it's better than nothing, but not doing the character real justice.
That's why Ridley as we've seen him is so impressive. Sakurai overcame that hurdle and made the character smaller, but still Ridley. The awesome grab move, the tail attacks, the design. It's all perfectly done. The character looks and feels like Ridley, but isn't too powerful as to be unfair.
But people watered down what he said to the "too big" meme and lost what the actual argument was. It wasn't about size as much as it was making the character balanced while still being faithful.