While Mary Barra probably has more on her plate than arbitrary and specific model support issues, it's still a bad analogy.
You can still get parts for a 1993 Cavalier.
And a '75 Monte Carlo.
Heck, my dad could drive his 1957 Bel Air into the Chevy dealer and get it serviced, like a new battery, if he wanted to.
(Source: employment, family in business, first-hand experience, internet forum comments)
In contrast, I took my wife's otherwise perfectly functional '09 MBP into the local Apple store where they declined to change out a bad battery because it was "vintage" or "antique" or "ancient" or whatever they call it. Absolutely no reason for that to be refused, other than servicing old products keeps customers from buying new ones. I can't believe there is any other reason the new Macs are as sealed up and as unserviceable as they are. The parts are still around and easy to make (and over charge for), the service itself it pretty easy (compared to a new macbook), and Apple store service rates are expensive enough. (See automotive example above.)
I've heard others opine (arguably) and I agree that part of the reason for the PC sales slowdown is because the tech improvements don't improve the user experience, if that experience is limited to email, web surfing and similar light duties. So how else to keep the purchases up? Make throw-away products. Cool for the investors, uncool for the customers - especially the ones like me who got on board with the (waning, if not dead) "think different" mantra.
The emperor's new clothes, indeed.