That aspect is the same as PoW. PoW rewards also depend on what the value of the coin does. You are right that you wouldn't normally consider alternate investm

That aspect is the same as PoW. PoW rewards also depend on what the value of the coin does. You are right that you wouldn't normally consider alternate investments like that. But with PoS, where the only cost is the time value of the money staked, it is the only thing to compare.