I think that's true for all the market, not particular to Mac. I don't think most people usually keep their computers for more than 2-5 years.

You're right that some PC brands also have high turnover, but remember that people are buying those PCs for $300-$500. Most people who invest $2k+ in computers keep those machines around for a while; even if their high-end usefulness has expired by year 5, there're lots of medium-end and low-end applications that can continue to be used on high-quality hardware.

It's still kind of new for a lot of products. Macbook Pros that were released before the Retina MBP (16 months ago) could be upgraded.