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 came 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.