Part of it is probably an attempt at reducing support costs by getting everyone on the same system. Also encourages developers to write for W10 if there is a large user pool.
They are resorting to the sort of tricks porno pop-ups used to use. This isn't just another upgrade that they hope will get a user base; heck, in the past they knew everyone would over time. But now they are being intentionally deceiptful and trying hard to trick users into "upgrading" against their will. Something is up here.
You do realize that Microsoft has to stick to the years of support they said they would maintain from when Windows 7 and the others first came out, right? Businesses and consumers bought with those years in mind.
Pushing people to 10 does not help them in any way. Keeping 10 and continuing to upgrade that will be what keeps the support on Windows down, over the years, though.