I'd rather that Apple deprecates older models of hardware than insists on supporting them perpetually. I say this as a developer. If you make an app for 10.7 you will not have to worry about building a PowerPC version, about testing it on a 2005-vintage Mac Mini just because one of your customers might have one. It's not supported. Period.
I know several people that are perfectly happy with an older system, 10.4 or 10.5 vintage either out of necessity or out of indifference. It works for them and they see no urgent reason to upgrade.
How much garbage do you think there is in Windows to support devices that haven't even been manufactured in the last ten years? How much better would Windows be if they abandoned all that and focused on making it better? I think it would look a lot like Windows 8, actually. Faster all around.
And what if the SSD or memory is not large enough anymore for your needs? (Which often happens after a few years of use.) In most other laptops and non-Retina display MacBook Pro's one can replace them at market prices. Not so with the new retina display MacBook Pro, MacBook Air, etc.
A friend of mine recently asked a quote for upgrading memory in his (pre-aluminium) Mac Mini, it was over 200 Euro. The market value of the memory was 40 Euro.
Edit: I would like to add that I am a happy Mac user. But I think Apple is rapidly moving into the wrong direction with their changes to rapidly deprecate older models, either by making them non-upgradable or pretty randomly removing support for them in new operating system versions (both OS X and iOS).