I’ll agree with this, but I don’t think it’s just about player population, at least not on its own.
Forums just aren’t the go-to medium for talking about a game anymore. Discord Servers exist. Reddit exists. I’ve seen so many old forums I used to frequent shut down and just have a link to their Discord on their website now.
The forums at this point are just a bunch of oldheads that got used to this place when it saw more traffic. It’s an even smaller fraction of the actual playerbase than it was a decade ago.