To me it seems that already happend, several years ago.Soon (if not now already) all normal people will stop posting here.
Already In the last 5-8 years many games were discontinued.
Also look at the decline of the page-count of the yearly random-WIP threads. It is not a recent trend.
BA10 never used the forum much.the BA debacle, how afaict that has catapaulted everyone with connections to BA 10 off the forums
From the beginning they had pushed for discord.
It is however a good example for the "broken windows" problem:
When gameplay of BA was discussed, too many people who did not even play BA raided the discussions. Despite not playing BA, they still wanted to give their opinions on it. Or they posted how in general gameplay/balance/updates should work.
Or they just wanted to say that they do not care.

That made discussions tiresome and in the end the actual BA players just stopped posting.
Or take the posts by Forbording Angle and PicassoCT.
The first repeatedly made wrong assumptions about how BA's gameplay and engine code worked. He vandalized threads by abusing mod-powers. (for some reasons he could moderate in the BA-subforum)
The second posted gibberish jokes or insults, I honestly do not know what he means half the time. Both made forum and discord unreadable.
Actually this not limited to BA threads, much of spring is very straining to read because of similiar problems.
The #main chat of the lobby died due to similiar problems when zeroK automated their advertisement spam.
I have heard that the #moddev channel suffered a similiar fate when the shitposting and memes took over.
Was is it ever tried to moderate these chats?
You make it sound like there is a contradiction. To me it simply reads as if moderation was not neutral.
Who would those figures be and what should they say?I wish I could have heard from some of the developers and other current major figures in this community.
I look through the list of Spring games and notice: Except for Metal Factions and Tech Anni, all are dead. Their developers left long ago.
The interest in those games is minimal on the forum. raaar is posting all alone in his subforum since the beginning. Tech Anni is considered yet another "cancerous alphabet soup mod", why should those players or devs be active in the forum?
Instead of supporting the active games, there was lots of talk about already dead games or vaporware. Or BA.
Similiar, engine devs left or never were very vocal.