The market did not moved to mobile. Mobile is alongside desktop. People like diversity and mobile offered another one. And people who have only mobile in their homes are poor that can't afford desktop but I see mobile as a gateway drug to desktop. Once people will have enough money, will also buy desktop exactly because mobile hooked them in the first place.
You can't really use percentages to say that something has declined if the whole pie is growing, and web use is maybe the one thing that looks least favorable for desktops.
That's a tale of saturation. Toaster sales are probably relatively flat as well. Not because toaster use has declined but because people already have a toaster that meets their toasting needs. PC improvements have slowed to a crawl over the past decade in terms of real-world performance gains for the average consumer. Their 5 year old rig still manages to surf the web, stream videos, play games and send emails just fine so they see no reason to upgrade. Another 4 cores or the jump from 14nm+ to 14nm++++ isn't justifiable for the average user. That said I bet the combination of COVID + AMD really coming out swinging the past 2 years might make for a bit of an anomalous spike.
1) Yes mobile moved the market by capturing hundreds of millions of people who would never have owned a computer otherwise, and Microsoft failed to react to this.
2) Do you have any data to back up the claim that mobile usage leads to desktop purchases? I have never heard such a claim before
Given that I see more Windows laptops and 2-1 devices than Android tablets around here, Microsoft has reacted quite well to this.
Additionally given the amount of people that run Office on their phones, in spite of total lack of usability to do such kind of work on those devices, again very good reaction.