The original question is, why upgrade to 10? To me that depends on the type of user you are.
There's a type of user for whom new technology, whether it's hardware or software, is not just another business activity, it's a major event. I call this kind of user The Enthusiast. Enthusiasts can't wait to get their hands on the latest shiny new gee-gaw, and spend hours poking around in and under the hood delightedly learning the fine new wrinkles the engineers have spun out of mere ideas.
To Enthusiasts, a new feature or a subtly wrought change to an existing capability is almost like a personal message from a software developer that says, "See how clever? See how elegant?" It's an appreciation of intellect, like a good story or a well-told joke, and provides more than sufficient motivation for the Enthusiast to adopt new technology long before more cautious users would consider such a move prudent. The Enthusiast is fully aware of the privacy considerations and the mandated update controversy and simply doesn't care.
These other users are more utilitarian in nature, and consider technology simply as a means to an end. Perhaps they run a business that depends upon a well-understood workflow with a stable infrastructure. Or perhaps they are by nature uninterested in the technological stories embraced by Enthusiasts. Whether or not they run a business, since they share the same mentality about technology, let's call them Businessmen.
Businessmen dread new software releases and question the value of every subtle deviation from the status quo and see these deviations, often correctly, as threats to their productivity. Sometimes Businessmen are simply not comfortable with technology in general, but often they are and are simply impatient with anything that impedes the accomplishment of The Mission (whatever that may be.) Pragmatism is the rule of the day for Businessmen. The privacy and updating issues are examined in the light of value received, and if the system provides enough value, these kinds of conditions are generally acceptable even if they are not terribly welcome.
To complete my gross over-simplification of the complex panoply of humanity that is the computer-using community, let's finally consider the Religionist. The Religionist views technology through an obscuring ideological haze that subtly alters the nature of reality. To the Religionist, the motive behind the production of technology is much more important than the actual technology.
Faced with an offering produced by talented engineers designed to delight their customers, they see an evil corporate plot cunningly crafted with the intent to enslave and deprive the masses of their... um, well, their whatever. Legitimate commerce is always seen as exploitation and enslavement, and the idea of consumer choice is considered a laughable charade to confuse the masses. This perspective is unassailable by any means currently known to man, however, it never prevents the Religionist from acquiring the latest technology, which they then commence complain about exhaustively. Religionists can often be recognized by the presence of a largely unused Linux distribution on a separate boot partition.
So, to the point of the thread, Why upgrade to Windows 10?
If you're an Enthusiast, the mere presence of Windows 10 is answer enough. You know what to do.
If you're a Businessman, the answer is, you probably shouldn't right now.
If you're a Religionist, the answer is to consider changing your medication.
Carry on.
