Well it's obvious that the decline started because the forum lost an important aspect of it, which has existed for long.
The mixing services. Fact is if you check on the month then decline began, which was in December 2023, it's the exact same month when theymos announced that mixers are now banned from the forum..
.. we all hope for a better future of the forum, so the hard choices had to be made.
Specifically, the lost of signature campaign for mixing services. Signature campaign is the main factor why people post here more than their average post per day.
Mixer campaign dominates the forum before the mixer ban so it’s noticeable the decline of post count.
Maybe good things will happened next year since @theymos already show a potential reconsideration on mixing ban.