They don't refresh the hardware aggressively because they just flat out don't need to. It's not about running down the Mac, if that was their aim they wouldn't have spent tons of engineering resources on the redesigned super-slim MBPs with new keyboard design and brand new whizzy touch bar. Those features may or may be problematic or even flat out mistakes, but they're not a sign of lazy lack of focus.
The thing is the vast majority of Mac customers don't know what a clock speed is, don't know the difference between RAM memory and Flash memory, and don't care, and are buying these things by the truckload. Apple has always been tardy about hardware updates. It looks like right now they really have actually dropped the ball, and at last they recognised that with the Mac Pro. However all the actual evidence points to this is simply a miscalculation, not a conscious choice.
The thing is the vast majority of Mac customers don't know what a clock speed is, don't know the difference between RAM memory and Flash memory, and don't care, and are buying these things by the truckload. Apple has always been tardy about hardware updates. It looks like right now they really have actually dropped the ball, and at last they recognised that with the Mac Pro. However all the actual evidence points to this is simply a miscalculation, not a conscious choice.