Did FB chose to replace friends' posts with garbage, or was it that less and less people were posting, and FB had to replace the feed with _something_?

Visiting friends' profiles, they still seem to be posting but I rarely see them on my feed.

No I haven't got them muted or anything haha, and I can't speak for why the algorithm thinks I don't want to see the content. Maybe it's broken.

Looking for cause and effect in a feedback loop is a fool's errand

Those aren’t mutually exclusive options. Facebook wants to always have new things to show people so they stay on the site, but it was absolutely their choice to deprioritize your friends’ posts below advertisers and the “engaging” slop.

Some mid-level manager idiot's a/b test revealed that they could maximize engagement by showing more rage bait and less family. This increased revenue and nobody wants to suggest a change that lowers it.