Exhibit A is whenever a story about public employees makes the front page of HN (for example, the Detroit bankruptcy or the BART strike) and people start freaking out about train operators making $100K: "They don't even have college degrees, for godsakes!"
The car retains the majority of its value, they enjoy the car, they're not actually spending massive amounts of money like you seem to think they are. It's essentially a hobby item that they enjoy driving.
And it's only a proportion of my friends, some just get other cars that are a lot cheaper to run.
I remember checking how leasing a Porsche for 3 years would cost me less than half of the value the car would lose in that time.
In general, though, the best way to combat the depreciation of a car's value is to take out a long-term 0% interest loan, provided of course that you can get it. This works because (1) inflation reduces your effective net expenditure on the car; (2) you can take the cash you would have spent on the car and invest it instead; if you even conservatively get 3% returns a year, you've still come out far ahead.
Regarding (2), if you can't afford the car in cash, then you can't afford the car. That's always been my motto. Financing is a choice; it should never be something you are forced to do, unless this is your first car straight out of school and you just have no money yet.
This is really pushing it. Anybody buying a BMW on $100k in Silicon Valley while paying SV-level rents is a pretty big over-consumer. If you have a family on top of that, well... you won't be retiring any time soon.
I also find it odd that you jump immediately from "a car that doesn't embarrass them" to BMW. There's no in between? You have to go all the way to a luxury BMW to not be embarrassed by your car?
By all means, if you really value owning a BMW then get one, but unless you're making considerably north of $100k you don't have a strong case for not being an over-consumer.
You know how much $100k was when the term "six figure salary" started being used in the early 60s? Let me put it this way. The rich Donald Draper on Mad Men made $45k. $100k would have been more like $800k today. That was a CEO's salary. A "six figure salary" today is nothing like what it was when that term was coined.