Golden age of forums was more than a decade ago, so no wonder that bitcointalk userbase keep decreasing as I've seen the same thing on the other forums I'm using; old users quitting and new ones are attracted by other platforms.
Another thing that probably attracted more people to bitcointalk are altcoins; from my experience, new people tend to get into altcoins first (looking for quick profit) and then later on focus more on bitcoin. From what I could notice, bitcointalk stopped being a place where people come to discover new alts, therefore losing potential users who would later be more bitcoin oriented.
I'm not sure that anything could be done to completely reverse the trend, but some of the things mentioned in previous posts could slow it down.
This is what I have noticed also. Other forums I have been a member of have
(a.) completely died with a handful of users rattling around or
(b.) have significantly less activity.
I would say that Twitter and Telegram are attracting more users than forums
especially for the generation who have grown up with instant updates.
While I love BCT I think the one drawback in the eyes of a newcomer is "The Vastness"
of it, there is just a massive amount of information here both useful and worthless,
it can be hard to,
(a.) find exactly what you are looking for and
(b.) time consuming to verify the information.
I suppose in comparison to other sources (twitter, telegram) Forums are slow,
require more reading and input.
I'm not going anywhere though.