When I launched
https://bpip.org, there were over 300,000 active members. Now there are 57,000. That prob has a lot to do with it.
But what is the cause of this ? The forum owners should be interested in identifying the possible causes and taking remedial measures. Like suppose, the search engine rankings are affected, there should be SEO done to attract more new active members on the forum from time to time.
The forum owner should be interested in identifying the problems and creating a better, attractive environment for newbies but for some reasons, he doesn't care (pardon but that's truth). But there are many good members that try to create different beneficial tools, like bpip.org, merit bot, post archive and so on.
If post per day or per month is falling, that's not a huge tragic. I always prefer quality over quantity, I hope that quality doesn't decline with it. If bounties and ICOs were driving this forum, then it might be a good idea to create an NFT board additionally. Now this is the trend (but we will attract spam, not quality.