[...] fiat long term value is doomed to asymptotically approach zero, by design.
I respectfully disagree; it's just a choice of systems.
Fiat is an endorsement of "wise men" controlling the currency supply. Gold/bitcoins make it far more difficult for men to change supply at a whim.
Like dictatorships, fiat is "absolute power." A benevolent dictator, Plato's philosopher-king, can do a great amount of good. Problem is, most dictators are rarely if ever benevolent or wise.
Supply-constrained currencies are pro-cyclical, tending to magnify booms and busts. In theory, the central bank may work against this. In practice... men are never perfect.
It's a choice between two systems with imperfect outputs. I think a mix of the two is most optimal, so that you're not a slave to the whims of either practice.
