You measure it's value because you get more purchasing power over time, if your spending BTC you will naturally spend less sats over the years as more people adopt it, like I mentioned before, BTC in not priced in fiat, it's the other way around, we will naturally be pricing fiat in BTC until it is gone and irrelevant and every alt coin will be priced in sats, you can say one xmr is one xmr but the reality is you will only be able to trade one xmr for a certain amount of sats, you won't be able to swap one xmr for one BTC because no one values them equally