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I don't know that the stamp will never go up in price--I think the idea is that once you've bought it, you can always use it. That is, they can raise the price, but the stamps you bought earlier will still be good. I *think* that's how it's supposed to work--but I'm not sure. Two problems:
1. What are they gonna do about speculators who buy lots of the stamps at an early time, don't use them, and wait for them to go up in price to turn a profit by selling them again?
2. That last scene in Fargo, where Frances McDormand's character tells her husband that his painting is still gonna be used on the 3 cent stamp will be totally lost.
They already do sell first class stamps that have exactly the properties you are describing. I think they cost a little more than buying regular stamps, so I don't think there's a problem with people stockpiling. To make money that way, you'd have to pay more money for the first class stamps and then wait a huge amount of time for the price in stamps to go up to an amount that exceeds what you payed for. You might as well invest your money in CDs.