Would you want to install Mountain Lion on a five-year old Mac? Hell no!
Apple saw that hardware was starting to last longer and took steps. Part of their solution was to try to innovate. Retina screens, multi-touch trackpads and an increased focus on gestures to justify their use over mice, etc.. Another part was to drive costs, and prices, down. Besides placing price pressure on the competition, occupying a lower price-bracket means people are more likely to upgrade more often rather than trying to nurse their old hardware along for as long as possible. This is the good. The other parts of their attack on long obsolescence cycles are not so good.
All traces of easy upgradeability have left Apple's product line. You now need special tools to get into pretty much any Apple laptop, and they use propriety connectors for everything. Want to pop an off-the-shelf SSD into your Air? Not gonna happen. Yes, you can order Apple compatible parts and tools online, but this is not by Apple's design. They'd probably sue those guys into oblivion if they were really doing a lot of business.
Apple laptops feel great in the hand, as if they're built to last generations, but they're actually horribly delicate in some respects. The Air can be bricked by a mere drop of liquid in the wrong spot (this actually happened to me. One drop. I'm not kidding.). The proprietary screws and integrated battery mean you can't remove the potential to stop the damage before it happens and then clean things up yourself. It's a horrible feeling watching your laptop fry itself knowing it could have been saved if it had been designed with a removeable battery. You'll get no sympathy from Apple either. Their warranty does not cover spill-damage. When this happened to me, the repair bill was literally larger than what I paid for my air in the first place! Subtle hint?
Software glitches and poor support for older hardware are only the latest in a long sequence of Apple's moves to keep you buying new hardware. Have the geeks looked at your obsolete macbook, tsk'd, and subtly hinted that while you could continue suffering with that old beast, a new laptop would be very cheap and work much better!
Apple is not stupid. They're actually really freakin' smart. I'd be willing to bet OSX users replace their laptops, on average, more than twice as often as Linux or Windows users.