Apple deliberately combats stagnation of their market in several ways. Their fashion forward approach and sensationalistic marketing are one way to make people want to upgrade sooner. Lack of support for DIY upgrades, soldered batteries, and crippling the CPU are another. Aggressively driving prices down to the point where laptops are almost disposable is yet another.
Consider what happens when you bring your Mac into an Apple store with a bad battery. The laptop is quite probably off warranty, especially if you didn't buy extended Applecare coverage. You will be told the repair will take a while and will be quoted a price that is going to be distressingly close to the price of a shiny new mac, whose prices have likely dropped since when you bought your old laptop. You're already committed to spending some money, and the feeling that you can buy a new, cheaper macbook for not a lot more than the new battery, plus the instant gratification of walking away with a functional laptop now will put the hook in you.
People generally pick up Apple products and make the mistake of thinking they're built to last. They're not. They're built to feel good in your hand and be replaced within 2-3 years.