If the only value that exists is monetary, you could well be right.
But society is much more than an economy, and some things far more profitable than mere financial increase. Since Steemit is a social media platform, the society is the purpose of the platform. Seriously, without folks posting there'd be no reason to have mined those massive stakes.
The ~10% retention rate is execrable for reasons, and the profitability of the token to investors doesn't only depend on marketing, development, and the percentage of the pool that can be extracted with it. Folks trading recipes, posting pics of the birds they saw at the park, and crappy memes are an integral part of society, which if Steemit is to grow and prosper is what Steemit must foster.
There is a misconception that it is content quality that creates value of a social media platform, and the truth is that it is the quality of the engagement; of the societal vigor that is generated, that is far more valuable.
The rewards mechanism is most of why Steemit's engagement is very good. Stake-weighting and the concentration of power is most of why it isn't good enough to retain more than ~10% of accounts YOY.
You can have a social community and social value without giving money away (or nearly as much). Indeed in some ways it is better. There is some psychology research suggesting that introducing monetary incentives actually substitutes for and damages social values, sometimes resulting in worse outcomes.