It is a perfectly fair question. There does have to be some source of demand for the coin and it's derivatives, and it has to be enough to sustain the system even when the system overall is growing slowly or not growing at all.
So you have to think about 'why would people want to buy and hold Steem or Steem Power other than an expected appreciation in value?". In my view the perks of Steem Power are immensely valuable, and would be only moreso in a world where Steem is huge. But if you don't envision people wanting to hold it in that scenario, then you should logically conclude that it is not sustainable long term. It's still ultimately a question that will only be answered empirically.