The majority of iMac or MacPro/Book owner does NOT buy every two refreshes...it's more like every 4 years if that.
I know. But wouldn't Apple love that we do?
I still use a Macbook 3,1 which has just turned 50 months old
I also use a MBP 5.5, which is half that.
Really didn't see the need to get anything else, but i did upgrade them with max RAM and SSDs