I feel like we must eventually reach an age where people have to pay (significant money) just to post.

Why are social networks allowing people to just broadcast to massive audiences for free?

I’m curious if content would be more satisfying if only the most motivated people were publishing content and not just spammers spewing AI drivel to grow their brand.

Because content used to be expensive. User generated content was free and drove engagement that is people returning on site.

Now that content generation is very cheap, this might change... See marketing and moves by Twitter...

The social networks already sort of work like this though, right? The algorithms constrain your reach considerably. And then you can pay to boost your content. I live in a country where people still use FB heavily and you need to use it to get local info. Most items on my feed creep toward things I don't follow or from people I'm not friends with. However, my feed reverts to showing items I opted into if I "hide" anything I don't recognize and then refresh. That seems to reset the algorithm for my feed.

This only works if people have a limited amount of money. Since some people have pretty much infinite money, money is not the way to limit things.