"This is trivially true (all civilizations end, the sun burns out, nothing is permanent) but the relevant timeframe is usually one's lifetime, and the stability of the currency over that timeframe is more important."
Not sure it is Trivially, fait can go to zero, gold for example can not as there are other uses for it... Perhaps, like good design, the more timeless option might be the better:-)
You're paying that "interest" in exchange for a more stable currency" who pays for whom might be the better question with who is to benefit.