I think to get to a unit of account acceptability the volatility would need to below 10% annually, it would need to be in the trillions so that no one person or entity could move the market as they move in and out and that will take time.

When people only get a marginal increase in purchasing power each year it makes it easier to spend and easier to price things in bitcoin. If BTC pricing also doesn't make it hard to maintain your margins as a business and carry risk of not meeting your obligations people can price goods and services in it much easier too

Not that fiat wont stick around for a very long time even if we get to that point, people are creatures of habit and some of those with that habit need to die out first for a new one to take hold.