If people expected no gain in USD (or inflation) then they would not hold USD and instead choose assets or other investments.
If one does expect profits they can achieve that not just by BTC going up in value, but other currencies being inflated.
I myself hold bitcoin as a store of value and do not necessarily expect profits. I simply feel that it will hold it's value better than other things. I could be wrong, but that doesn't mean it is a Ponzi.