Assume the demand of Swiss francs is "x".
Assume the supply of Swiss francs is "y".
If the price of francs is higher than what Switzerland wants, then:
x = y + z
where z is the amount needed to match supply with demand.
In that circumstance, Switzerland can print z amount of francs -- that is entirely sustainable. And that is what it was doing for the last few years. Keeping the price stable is the same as saying that it was printing z amount of francs, so that y+z equalled x.
Anyone who says this was unsustainable is simply not thinking clearly.