" If Beehiiv is a Ponzi scheme then so are other similar platforms like the one you are on right here and now!"

Also known as multi-level Marketing (MLM). And Medium is no, like that. Medium members pay a subscription to Medium from which Medium pay its writers (you and me) according to how many members have read and shown their interest in / appreciation of your work.

From your description, Beehiiv members make money not according to the quality of the work they produce but off the backs of other writers they recruit as subscribers.