Macs have always had pretty good longevity, and that's been even more true for the ones produced in recent years. I usually get 5-6 years out of Mac, and even then can usually sell it for not too bad a price to someone who will surely get several more years out of it.

I would say that the likelihood that most of the growth in the Mac line in recent years has been "switchers"; now they're all enjoying a computer that can actually last you 10 years potentially, so sales of course will stagnate.