If you earn in fiat, why would convert fiat to bitcoin, and then pay in bitcoin? A rational economic actor is not going to do that.
So a bitcoin circular economy is not going to organically grow significantly until:
  1. Those who invested in bitcoin see gains and thus might spend some of those gains. Or,
  2. Merchants wanting to earn in bitcoin offer a discount for payment in bitcoin, and to a degree that customers will acquire bitcoin so that they can take advantage of the discount. Or,
  3. Price inflation or other factors push earners to acquire bitcoin as savings, and thus spending in bitcoin will be preferable to converting the bitcoin savings to fiat only to then spend in fiat.
There could be other contributing factors that would spur a circular economy to develop -- such as some type of economic assistance (e.g., UBI) paid in bitcoin. Or perhaps community banking like what Galoy is building becomes more popular. So that's kind of like an alternative to bitcoin that is not much unlike stablecoins. If you had a stablecoin or funds from this community banking, paying a merchant in bitcoin is easily done because the funds are already digital, and conversion to bitcoin at the time of purchase can be done quickly and cheaply.