Why does depreciation matter at all? A car that I buy brand new and drive for four years may well depreciate 25%. But that number means nothing unless you plan to sell at that point. If you're planning to keep driving the car for another 16 years, the number is meaningless.
This is a good point, but there is more nuance. Depreciation is somewhat indicative of quality/utility so often we substitute it for quality (but with vehicles this gets messy). So if something costs $50K one year and $25k the next, it is assumed that it has lost $25k of it's utility. But utility for the market isn't necessarily utility for a mustachian.
It would be easier if the value of a car didn't have so many things tied to it (luxury, reliablity, mpg, image, etc.). If it were pure reliability/function, then depreciation
would really matter in analysis because the price of the car would correlate pretty evenly with the remaining cost of ownership. But vehicle depreciation also incorporates the "newness" factor in it, which varies greatly depending on brand and person buying a car.
Another way to look at it is: would you get more utility from a $10K 2010 Honda Civic or a $10k 2010 Mini cooper? The Civic is objectively more reliable (all things being equal). If we are only looking at price, both options are the same. But if we look at depreciation, the Mini cooper has depreciated almost twice as much as the Honda since new. (maybe just 1.5 times if we go off of purchase price and not MSRP). A good portion of that depreciation is due to the fact that the Mini has used up quite a bit more of it's average lifespan as the Civic. But not all of it. An argument can certainly be made that good maintenance will more than make up for average reliability, but that takes us out of the realm of general data and back into anecdotes.
In general, depreciation is an indicator of loss of utility. Cars that loose 10% as soon as they drive off the lot are perhaps depreciating more because they don't have that "single owner" auroa to them. Some vehicles are worth more then next year when they are reliable and in demand (happens to Toyota pickups all the time).
I'm rambling here with no real point other than that we can't use depreciation as an end-all, but we can't dismiss it either in my opinion. Even if we're not going to sell the vehicle, it's current value vs original value is at least somewhat tied to it's remaining utility (unless we get into the realm of exotic cars an the like).