You again come up with mutually exclusive starting conditions

There is no good medium of exchange which would be as good a long-term store of value in practice. Basically, you have just invented a perfect money, and then put it against a real fiat currency which is always inflationary and thus can't be a good long-term store of value. Indeed, the fiat currency would lose and collapse, but this approach is meaningless as one can always imagine something perfect, not possible in real life