I think the longevity of XP was unintended. Remember, Vista was delayed for almost 2 years, and it wasn't well received.
During the PowerPC and Motorola 68K era, generally Macs had better longevity than DOS/Windows PCs. That changed a bit during the Intel era, and accelerated in the last few years of Steve Jobs, particularly as Apple's fortunes became increasingly tied to iOS, which in general is built upon a 2-year upgrade cycle (since phone contracts are 2 years long). Intel's switch to an accelerated timetable for new processor releases also has had an impact.
That said, there are practical reasons for the dropping of compatibility with older Macs. With 10.6, Apple cut the cord with PowerPC. 10.7 became primarily 64-bit, and so it was natural to drop support for 32-bit Intel Macs (unfortunately, the Core 2 Duo came out a few months after Apple made the switch and so the early Intel Macs had the older 32-bit Core processor based on the older Pentium M). 10.8's cutoff is slightly more arbitrary, but it appears to be tied to graphics capabilities. It's important to point out that the Intel shift, from a technological perspective, was a slight step backward at first. PowerPC had gone 64-bit sooner. The switch to Intel was driven by availability and power consumption.
