One thing to consider is that STEEM (in all forms) has inherent value within the network itself. STEEM Power dictates how much someone earns for their posts (and how much others earn when that person upvotes content). SBD can be used to "promote" posts within the network.

So, aside from market speculation, these two factors help the tokens retain value. Only time will tell whether that will indeed work, but it doesn't seem a whole lot different to me than the "money" of any other community, such as a country.