"But when steemers choose to purchase more steam power that doesn't directly benefit those who joined before them or those that referred them."

Sure it does! The whales benefit (earn) from investments and curating by the minnows for which they (the minnows) get paid nothing. Without so many minnows, there would be no whales.

"Same thing here - people may wish to purchase some virtual power with real money and they may not get any material benefits from it, but this doesn't make Steemit a Ponzi scheme."

I do not agree! People invest in a game solely for the Entertainment value that it provides. There are no other claims. People invest in Steemit, on the other hand, as a result of Steemit's claims to reward curators, and in the belief that they will actually make money, whereas in reality, they do not, or at least the vast majority do not. And that is what makes Steemit a ponzi scheme.

If rewards were shared equally amount all curators, that would be a much different issue. Likewise, if those members with higher reputations did not have any more downvoting power than lower reputation members, that too would be a much different issue. Downvoting other members simply because you do not agree with what they have to say, has absolutely nothing to do with maintaining a quality forum. In fact, quite the opposite is true. The quality of the Steemit forum is extremely low for that exact reason, because serious social media people will have absolutely nothing to do with a platform such as Steemit.

If Steemit was designed properly, it would have perhaps 100+ million members by now, but as it is, it has only managed to attract a mere 1 million, with less than 5% of those still active.