Steem has the value of 1.5 $ at the moment. That's determined by the market. If you write a post or reply and get upvoted, you get a certain amount of Steem. How is that a Ponzi ? If Steem is worth 1000 $, you get 1000 $. If it is worth 0.0001 $ you get 0.0001 $.
Concerning the investors, well, nothing is guaranteed in life. Those that seriously invest in here, they should also be writing articles and deliver quality. That's how they can make money. But every day people are getting Steem and this amount is linked to your Steem Power. So people with a lot Steem Power get more free Steem on daily basis. So that's an other way to make money. Could Steem go down and hurt investors ? Of course it can, just like it hurts investors that hold Bitcoin when the Bitcoin price drops.
So seriously, I don't see the Ponzi Scheme here.