Well stated! I agree wirh you on most of it The only point I want to make is:

the size of the userbase and the fact it meant you had access to literally any type of person at your fingertips

The niche communities themselves tended to be small and focused, which is what I say improved the quality of the content. Contrast with the large, default sub's when I think we both agree failed to add value. I say that communities happened to accrete there was because it was low effort and low friction. Now, not so much. It was a naked grab for cash by usurping the uncompensated efforts of a few dedicated people. The true believers moved on.

As a market place of ideas, reddit was a good mega mall. The anchors sucked but the boutiques were cool. Now it's just a great big building full of disregarded storefronts after the holiday sales have ended.