But that would be inefficient. Why re-buy the "wasted Bitcoins"? Why not merely use the fiat to directly buy the car instead of using your Bitcoin, then replacing the Bitcoin? That's why it doesn't make sense that users do "Fiat -> Bitcoin -> Purchase" unless you absolutely have no choice but to use Bitcoin, or if you truly earned the Bitcoin you spend, like Laszlo.
Of course the ideal situation is when you also earn Bitcoins, e.g. if your salary is partly paid in BTC, and/or if you are a service provider accepting BTC. Then the "Bitcoin circulation" aspect would come naturally: You could spend a part of the Bitcoins in things like a Tesla car, and you'll get them back (albeit getting back what you paid for a car it may be quite slow, but Tesla cars aren't directed at the lower middle class). I'm aware that this isn't the norm today.
Another case: Particularly in market situations when you have already made some profit with your BTC, it makes sense to spend part of them if you can satisfy a necessity, like a car. I think particularly producers of high-priced goods like cars can direct their efforts to that public: early adopters or those who are "in" at least since the bottom of the last bear market. You'll not always have (in addition to the BTC) the fiat to spend it on such a good, and in this case it may make sense to simply spend the BTC, and if you like to buy back some BTC, then you can do it.
I don't think "changing between payment methods" is that much of a hassle. People do it all the time, transferring between different bank accounts, using ATMs, etc, so re-buying BTC if you have a good exchange partner isn't much of a problem. And again: if you're a Bitcoin bull, strengthening the currency ecosystem is adding value to BTC, so why not simply do it? You can even get social prestige (e.g. posting your Tesla you bought directly with BTC on Twitter, pardon X).
And there is another aspect: Buying BTC has become increasily easier than selling them, due to AML/KYC regulations (proof of source for BTC you received, etc.). So if you have some BTC from old investments in ICOs or altcoin trading for example, which are difficult to track because the exchanges where you traded don't exist anymore, then a purchase of a good may be a good way to spend them. That may be not the case with cars as car sales are regulated in many countries, but with other goods it isn't a bad idea. The re-bought BTC instead can be tracked well and it will be much easier to provide a proof of source for them.
But given the context, it's not about "changing payment methods", nor is it about "the increasing ease of buying Bitcoin today". It's about efficiency. Why would I, a fiat earner, buy Bitcoin merely to use it to purchase a car? I could buy the car using my fiat directly. Why the few extra steps unless I'm incentivized to do them, like perhaps a 30% discount on the car if I use Bitcoin?
A case when a person has no choice but to use Bitcoin is if his/her computer is compromised by ransomware which requires payment in Bitcoin. Or if a Heroine addict can't get Heroine anywhere except the Dark Markets, would be another good illustration of payment in Bitcoin. Another case would be if someone doesn't have a credit card, but he/she wants to gamble in an internet casino. But Tesla, or an iPhone, or another common item that can be purchased for fiat, those markets are very efficient that there's no need to use Bitcoin.
