There are always marketmakers on the exchanges who buy to facilitate trade. If you look at the poloniex wallet, they hold a huge amount of steem, which means they are acting as market makers rather than speculators (if speculators were buying, the steem would be in their account). See

https://steemit.com/@poloniex/transfers

You are correct that minnows and dolphins are accumulating steem and steempower - but this is from newly generated steem. I think about 800 steem are generated a minute, and of course voting distributes that new steem. The redistribution isn't enough to sustain the price, because you need new buyers to absorb the steem already generated - that is why we need speculators. All currencies depend on speculators even fiat!