I read a good article about this on here the other day. Basically, you don't have to buy in. You could potentially just earn your steem & steem dollars.
I mean, well, it could still collapse, but a true ponzi would have everyone buying into it. Really, I think many are just investing time &a effort...
Fair enough, but my point being where is the money coming from to pay out to those who don't buy in and just earn, it's still from new money coming in.
If you search here you should be able to find better explanations than I can give.. Basically new steem is created daily, overseen by our all mighty witnesses..