They are in decline. They are dying.
That's the fallacy people fall for, including you: being in decline in no way or form means that it's dying. It just means it'll settle in at a different place than it was before, likely a smaller place, and then stabilize there. Dying would mean exactly that; the whole desktop-PC market disappearing altogether. Alas, there's plenty of people who still need desktop-PCs for all sorts of things and then there's the people who don't specifically need one, but who find mobile devices simply too clumsy and cumbersome to use.