How would Bitcoin's purchasing power be measured after fiat were gone? Somewhere in the possible near/far future when there might not be any major fiat anywhere. What will Bitcoin's purchasing power be measured in?
It's typical to measure the purchasing power of currencies using something like the Consumer Price Index, so this is probably would be used if Bitcoin were to become a dominant currency.

Basically, you would develop a "market basket" of goods and services that people commonly buy, and perform some sort of survey to determine what these goods and services currently cost  (in Bitcoins).  Then take some weighted average to come up with a single number that, roughly speaking, measures the overall Bitcoin price of goods, or equivalently, the purchasing power of a bitcoin.