The 'all you need' part is the part you can't hand-wave away. One typically can't both satisfy the requirements of generating a rate of return needed to remain viable[1] with low risk and keep the funds liquid. In the event of a run on their currency, they would need to have most of those funds available on short notice without a significant loss of principal to avoid an implosion. Even liquid, safe government bonds will often need to be sold below the price paid during extreme market conditions (the exact kind of situation that runs on the coin are likely to occur during)

[1] Especially in a near-zero interest rate environment.