The problem is you are making the assumption that you'll be able to cash it out in 2 years, and while I'm hopeful for this platform, it has yet to proven itself a sustainable for long term.
It's a matter of risk. For some people, getting thousands in steem dollars is a lot of money and might not be in a financial position to lock it up for two years, it'd make more sense for them to get the guranteed payout.
I'm not convinced yet about Steem enough to buy lot of SteemPower with my fiat money. But I would convert all my Posting Rewards to it, because I hope to make more cash with content at Steemit in the future. And thats why I try to find a rational way to prove that Steem has an inner worth. What about the worth of the social news site Steemit.com? You can measure it (unique visitors, pageviews). Couldnt this backing the worth of all the Steem?