Well first they are selling that many Unit, so it is either their user replacing old unit and no user growth, or their 50% of Mac are to first time user stats are bogus.

Second there is still over a billion Windows PCs on the market. And judging from Intel's latest quarterly results suggest the PC market is still in very healthy replacement cycle.

I know a lot of education are going all in on Tablet. I do wonder how big the market is. I dont have any Data with that.