This price volatility comes from comparing Bitcoin with Fiat. If Fiat doesn't exist and all the people use Bitcoin as the only currency, A pizza always costs you 0.002 BTC. In that way we will use Bitcoin as same as we use Fiat today.
This is the premise of the original qestion. How does the world make the switch from fiat to crypto (in 10, 15, 25 years if ever). What does that look like? Does a single nation need to give up its fiat currency and only exist in BTC, and hopefully start a domino effect?