I have been here for 15 years now.
I have been other places, on and off, for varying shorter times.
I now spend my "forum time" helping a couple businesses that engage in other ways and primarily attract a younger demographic.
My opinion, especially with my day-to-day exposing me more to Zoomers and Millennials in the gun world, is as follows:
Younger generations see forums as antiquated repositories of Boomerisms and far too slow for the immediate answer that modern technology allows them to get on other subjects.
They do search forums and read posts as "guests", because they know that useful information is buried within.
But they don't like taking part and don't sign up, unless to get some photos that they saw previews of in a google image search. They hate wading through the Boomerisms and ridiculous 5-page posts about some particular member's history with [X] rifle, just to see that they never answered the question, "Should my trigger have a little horn here? This one seems modified."
They hate asking a question, only to be berated for buying a stupid gun that they knew was stupid.
They hate asking a direct, pointed question, only to see zero replies for weeks, because no one else ever stepped back to ask, "Why is [that] the way it is?"
They hate the rants. They hate the tangents. They just hate forums, because they represent Boomers. And Zoomers (and many Millennials) just hate Boomers.
They are using other things now. Slack, Quora, Slingshot, Mumble, MeWe groups, Discord, Facebook groups, Twitter, Reddit, and more.
All of them have better response times than forums (even if the information is false and worthless), and many offer the equivalent of 24/7 live chat -- because that is pretty much what some of them are, just live, logged chat.
As Boomers die, there is no new blood to replace them on forums.
Boomers, Gen X, the lost generation, and Millennials, in their respective numbers, are all that is left.
As we leave, the numbers just dwindle. Zoomers rejoice as forums implode, disappear, and take with them decades of valuable information (but also a lot of garbage).
Zoomers are driving current trends in group communication, and they just hate forums.
It does not help that a huge number of popular forums have been sold to corporate entities in the last 5 years, resulting in the use of new forum software that has even more tracking abilities (bad - Zoomers hate this more than we do) and forcing even more ads down their throats - even if paying members of some special rank that reduces ads.
Zoomers hate forums. So the active member number will dwindle as the number still alive follows the sled ride down the back side of its Bell curve.