When you earn in bitcoin you will then opt to spend in bitcoin. Fiat then is reduced mostly to unit of account (i.e., prices still set in dollar (or other local fiat) terms, at least until the local currency is as volatile as bitcoin and then fiat disappears as the unit of account as well.

But what will be the catalyst to cause us to demand to be paid in bitcoin? Freelancers, sole proprietors, entrepreneurs, etc., have more leeway to begin the process of persuading customers to pay in bitcoin. Wage earners who receive a paycheck from their employer will have a harder time -- though there are now methods where the employer doesn't even need to do anything different (e.g,. BitWage gig worker mode, Deel, Hedge, Juno.Finance, Strike's Pay Me In Bitcoin, etc.)