With the actual nature of the market, I'm not sure this is as significant as people will think ...
I have a 5 year old laptop that, other than the $25 battery I had to buy, still works really well for what I use it for and a 18 month old tablet that (after a few OS updates) is now painfully slow to simply browse the web. I have noticed that people are replacing their tablets every 12 to 18 months, while their desktops and laptops are being released every 5+ years; and, while I have seen no studies to demonstrate this, I think this is far more common than people would assume.
In other words, simply outselling them doesn't mean that tablets have replaced laptops