Microsoft need to maintain a revenue stream, its as simple as that.
I suppose the obvious analogy here is car manufacturers. Cars are designed to go 10-12 years now and then fall to bits, or more accurately now, electronically "die". Unservicability becomes regular, spares and repairs get increasingly time consuming and expensive. Eventually people just get a newer vehicle to stop the aggro.
Unfortunately many modern businesses have cottoned on to the concept of customers having to buy a 'subscription' to a product or service (much easier to keep tabs on people in this internet age). I suspect in the long run this sly marketing technique will yield them far more revenue (and the customer more expense) than just selling a product as a one off.