I am not theorising about their costs, rather, I am referencing their own financials. Per their financials, they must achieve >$4,500 in gross profit per car sold in order to break even on a sale because the cost of providing their service is around $4,500 per car. As far as I know, they have never achieved this and they have only come close during periods where the second hand car market was at its most ridiculous supply-chain induced peaks.
You could start an online second hand car dealership today, with the same online purchasing that Carvana offer, and a better quality of service, and build a great business with great unit economics... if that was your strategy from the first day. Many such companies exist! There are lots of upmarket car dealerships that'll give you excellent service -- and it'll cost you less than $5k over market.
Carvana can't just undo a decade of bad mistakes, they can't just switch from "losing money" to "making money" because every foundational decision about the business was made in the context of "who cares about money, we want growth". You should not think of a company as an implementation of a business model, a company is so much more. Carvana has something like 20,000 employees: at that scale, radical change is basically impossible.
At its core, my argument is that there is a sweet spot in which you can get great service from these "boring industry + technology + money" companies but that's because they're actively pursuing good customer service at any cost with no consideration for making money. As soon as the company (or its investors) realise this behaviour is unsustainable, they forget all about their narrative and go all in on making money -- which is when the quality of the service slips.
The amount you pay to Carvana (more or less than you'd pay elsewhere) doesn't have a relationship to the quality of service you will receive, because it's not a "normal" business.
There can be positive consumer outcomes from companies that approach business like Carvana: in the short term, customers get investor-subsidised products, and in the long term, other players in the industry get to learn from consumer reception (e.g: CarMax probably learned about demand for higher quality service from what Carvana demonstrated early on) but Carvana specifically is doomed to failure because it isn't a sustainable company that made a bad decision during the pandemic... it's been unsustainable its entire life.