This is a somewhat theoretical discussion but I am of the opinion that especially in this case it is demand that attracts supply and not the other way around.
The crucial question is however what attracts demand (by advertisers).
Nowadays a new cryptocurrency exchange, to give an example, is going to get a lot more customers if it advertises on a Youtuber with millions of followers than by advertising on the forum.
Even if the traffic of Bitcointalk is of course lower than the traffic/followers on the "crypto YouTuber" scene or "crypto TikTokers", the equation is the same on YouTube/Instagram/Tiktok than on forums. On big social/video platforms reaching out to a large number of readers/followers is not cheap.
(edited to make the point clearer)In addition, a forum has some advantages with respect to particularly video platforms. Videos often get most of the views in the first days, while forum threads have often a longer life. This is of course not true for spam megathreads, where people only view to post (and they will only view the last page). Spam megathreads are thus a substantial threat to this advertising model, and it's quite good that their importance is slowly decreasing. Video ads are also often more easily ignored (by fast-forwarding, if the ad is too long) or blocked.
There is also link building and brand building. Link building is one of the few cases where even spam may work for advertisers, but its importance has declined in recent years as search engines have found ways to deal with the link spam problem, which of course is not limited to Bitcointalk but the whole web. Brand building is in contrast benefitted by high quality content.
I also have observed that forums were declining mainly in the period between 2010 and 2016-18, then they seem to have stabilized again. In the last 5 years I saw not much variation in market share between "forums" and "big centralized social platforms". It was more a shift from Facebook to Instagram (in-house) and TikTok, and it's possible that the decline of Twitter/X in recent years is a chance for more decentralized platforms, including forums, to grow again.
This seems also to be confirmed by the
ad stats I linked above: reader volume (total impressions & unique IPs of logged-out users) even grew since 2022. Only during the bull market peaks in 2017/18 and 2021 the activity was notably higher, and if comparing the 2018/19 crypto winter and the the 2022/23 crypto winter these numbers didn't differ substantially. Logged-in users have declined a bit but not catastrophically, this could be an effect of the mixer ban.