Given that most of us believe that Bitcoin will increase in value over time, you will want to spend it instead of Hoddle and let it appreciate.
Example: Suppose I bought a car with 1 Bitcoin today, but the value of Bitcoin doubled next week, which means I'd better use fiat currency.
Fiat currency or Bitcoin or cryptocurrency is only one of payment method you use to buy something, a car in this case.
If you spend your money to buy anything, it goes out of your pocket or wallet.
Your assumption is not make sense. Because if you only have Bitcoin, and want to buy a car, you have to sell your Bitcoin to buy that car with fiat currency or directly use Bitcoin to buy a car (if the company accepts Bitcoin).
If you believe in Bitcoin, and don't want to sell it to buy a car, hold it.
If you have spare fiat currency and big enough to buy a car, use that fiat budget, instead of Bitcoin.
I guess your assumption is for people who only have Bitcoin, and have to choose between holding it or selling it to buy a car. So tough choice has to be made: keep your bitcoin, forget about that car for a while; or sell your bitcoin and get that car. A binary option.