For any currency or money there is a universal truth. They only have value because people think it has value. Even government backed currencies eventually have to hold to this truism.

Bitcoin has its inflation scheme (aka block-reward) system set in such a way that early adopters are punished, causing the value of the coin to not grow exponentially. With the vast majority of the coins having been mined that will ever be mined, this no longer applies.

Steem doesn't stop charging holders, though. The inflation isn't there to "kickstart" the currency, it is a loop in and of itself. Not the same thing.
In Steep the holders see their value diluted so new content creators can get paid. And in exchange for them holding they get voting power.

I would agree that Ponzi isn't appropriate in the original sense for either.

What Steem does is it creates a platform where people with more holdings can decide what others read by up/down voting.

I'm still on the fence if that is a good thing.