I agree with you in theory, but people don't know the value of an item even in this purely bitcoin economy because it's not a stable store of value, so they have to consult in terms of USD still to see if the value of btc is falling or rising relative to fiat currency. Because btc is not a stable store of value, I don't see "defiatizing" the unit of value as possible.
Take the people in Japan for example, their currency is very unstable and lost 50% of its value against dollar in 2 years. But in Japan, a hamburger used to cost 100 Yen would not become 150 Yen now, but still cost around 100 Yen (maybe 103 Yen since their central bank said the inflation is less than 2% per year )
Similarly, if a burger in bitcoin economy cost 0.01 bitcoin, it does not matter how much bitcoin worth in other currency, as long as everyone only use bitcoin to purchase the burger at a price of 0.01 bitcoin, the bitcoin's value will be stable
The problem is that currently no one use bitcoin to measure the value, while most of the people use fiat currency to measure the value. The reason? A consensus has not been reached, the bitcoin is not a currency that is forced upon its user by the government, thus the consensus of its value can only be derived from its cost, not like fiat currency, universally circulating in the country thus make it much easier to reach that consensus of its value
FED has printed 6x more USD since 2008 and you don't see anything's price rise by 6x, and you don't see GDP increase by 6x either. But people still think USD's value is stable, since they have a consensus: Today one burger cost $1, and tomorrow it will still be $1, it is that consensus holds fiat currency's value, so that no matter how much currency the FED print, people would still think one burger cost $1.
Some people who learned economics will understand that now USD should only worth 1/6 of its value since 2008 and everything's price should rise by a couple of times. But when they go to the street, they will find out that no one believe them, and almost everyone still think dollar is almost as valuable as 2008. So once the crowd has reached a consensus of USD's value, it will force anyone else to accept their valuation of USD (since they price the same thing using the same dollar amount as 6 years ago)
Similarly, if there is a large enough crowd reached a consensus of bitcoin's value, then they would also force anyone else to accept their valuation of bitcoin through a fixed pricing of their goods/services