My forum, which is 20 years old and has 738.5k posts, makes me earn much less with Google Ads than my WP blog which has 50 articles...
You seem to be focused on money, as you have mentioned several times in this thread. Money is not a factor on whether a forum is dead or not. Many forums don't monetise to begin with.
Facebook has a unique purpose and some groups go well, most do not.
You never own the content though, which is huge, and monetisation of a FB group is not as simple, and certainly nothing unique that you can't do on a forum. Example:
https://www.uscreen.tv/blog/monetize-facebook-group/
So really... if you are talking about monetisation, then really, an FB Group is just a lead generator to push users elsewhere to extract revenue from them, because you won't get much from the group setting itself, controlled all by FB. Doing that is enough to lose a whole bunch of your user base, because many people like something until someone sells out and their space is suddenly filled with advertising.
Forums have a time and place. FB groups have a time and place. Sometimes, a single website is best when using a combined approach.
People have been saying forums are dead since FB came about, and then groups, and then Reddit, then blah blah... and yet, they're very much still here and active. Sure, some died... but the reasons can be various, and FB can be that reason for some, but the owner or mods can be a reason a community dies too.
Running forums for over 20 years, content is king to keeping a forum alive and well. The moment you, as the creator, take your finger off the trigger, the community will decline. Which is akin to saying, the moment you lose the passion for that community to exist, is the same moment it begins its decline. A community will not typically build leaving it to others alone.