My big take away is that social media as we know it is likely generational
I don't think that's the right takeaway. The demographics of certain platforms may be skewed, but people who for example were active on Facebook 10 years ago still exist, they're just posting a lot less.
I think engagement is down across the board because of various reasons: the continuing crappification of the various platforms, people are starting to realize the risks of oversharing and public sharing, people are getting turned off about loud toxic discussion, people are becoming aware that their data is being mined by faceless corporations who don't have their best interest in mind, in short all the negatives of these platforms have become more obvious to the average user.