Been here since 07-ish only browse occasionally nowadays. There was definitely an enormous shift in general culture post mobile phone revolution, and then again with the election tourism, in which every edgy 16 year old in the world was advertising 4chan on their socials to spread hekkin based Trump/Kek memes.
One nice thing about 4chan getting taken down for a week was that all the board stats got exposed. This site has definitely declined severely in activity, and looks lower than the stats that moot talked about during one of his send off podcasts, which is pretty grim.

Basically, election tourism spreads like wildfire, /pol/ shit up every board to the extreme, a ton of people leave, election tourists eventually leave, neither come back, you've got what we have now.
Also, the nature of the boards are just different. Most discussion about anything serious is relegated to generals a lot of the time, pretty sure 90% of /x/'s daily activity is in succgen, nobody gen, and the christ-cuck gen.

As for your "theres just less people alive" theory, its factually incorrect. There are more real people on the internet than ever, its just that 4chan's old culture is gone, and the people left with it, on top of that the vast majority of real people on the internet congregate on like one of 3, maybe 4 sites.

In a way, when you think about how congregated in a few places everyone is, plus AI and botnets running rampant over everything else, the internet at large really is dead.