> Many people are easily making 6 figures on Substack now

How many though? I get the impression it's really just a very small subset at the top, with a very long tail making almost nothing.

Isn't that basically the story of 99.9% of any sort of Internet publisher, e.g. I imagine the same dynamics apply to YouTube or Twitch. It's a fundamental feature of the "winner takes all" economics of the Internet.