Because by spending bitcoin you are actually fueling the bitcoin economy, you are showing interest to the sellers and so the price of bitcoin will go even highe

Because by spending bitcoin you are actually fueling the bitcoin economy, you are showing interest to the sellers and so the price of bitcoin will go even higher (companies see the interest -> start supporting it -> start buying it -> start marketing it -> more people using it -> price goes up). Further, if you believe that bitcoin is going to be used as a currency at some point, you may as well start using it already. And finally, since bitcoin is so liquid (you can literally buy or sell in seconds with custodial wallets/accounts like Strike, or in minutes with self-custodial/private solutions), it doesn't really make sense to "sell the worse currencty (fiat) first" you can just buy whatever is your target amount of bitcoin today. You don't need to wait. This is not physical gold that takes hours up to days to buy or sell (you have to go to a special place, etc...). Some people do "spend-and-replace", where they spend bitcoin for products/services and immediately buy the same amount of bitcoin for fiat. That's a nice way to start, but it's sort of unnecessary.