There's nothing there that is impossible without a blockchain. The steemit website might as well issue its tokens using a simple database. Running it on a blockchain adds nothing except a 'shiny new technology' feel.
As a user (writer/curator), you earn tokens for doing what you already do for free in reddit or facebook. The tokens you earn are like altcoins, you can trade them for whatever you like.
As you say, the key point of blockchains, like bitcoin, is that there is no central trusted entity. So, like steemit has done, if you add a trusted person or company, the main feature of the currency is wiped out and there's no point in having a blockchain in the system.
I'm not saying this model is foolproof, and perhaps you're right and steemit (or app specific coins) will never be a big hit because of that. My point was just to show an example of a different kind of business model / monetization strategy made possible because of the blockchain, and there will certainly be others.