Try putting a bigger SSD in your Air sometime. It's possible, but far from easy. Even the Macbook Pro's are becoming more difficult to upgrade with each new generation. The Retina Pro is rather similar to the Air in terms of ability (or lack thereof) to upgrade.
Also, your 2008 macbook probably isn't worth as much as you think it is. Over the last few years Apple has done an excellent job of driving their prices down. A typical macbook pro from 2012 is more than 25% cheaper than one from 2008, if you're okay with a smaller screen an Air would be lighter, faster, and even cheaper.
What is true is that hardware requirements, in general for all OS's and software, have been stuck in neutral for the last few years. Whether your computer is a desktop Dell, a home-brew gaming rig, or a Mac, you probably haven't really needed to upgrade for a period of more than twice as long as you would have even just five years ago. That's why your 2008 Macbook doesn't seem like a hunk of junk, not because it's particularly powerful. While other manufacturers are being bitten by the slowing obsolescence cycle of computer hardware, Apple is very deliberately speeding the obsolescence cycle of their hardware up with a combination of marketing, reduced ability to upgrade, and price reduction.
Also, your 2008 macbook probably isn't worth as much as you think it is. Over the last few years Apple has done an excellent job of driving their prices down. A typical macbook pro from 2012 is more than 25% cheaper than one from 2008, if you're okay with a smaller screen an Air would be lighter, faster, and even cheaper.
What is true is that hardware requirements, in general for all OS's and software, have been stuck in neutral for the last few years. Whether your computer is a desktop Dell, a home-brew gaming rig, or a Mac, you probably haven't really needed to upgrade for a period of more than twice as long as you would have even just five years ago. That's why your 2008 Macbook doesn't seem like a hunk of junk, not because it's particularly powerful. While other manufacturers are being bitten by the slowing obsolescence cycle of computer hardware, Apple is very deliberately speeding the obsolescence cycle of their hardware up with a combination of marketing, reduced ability to upgrade, and price reduction.