I don't think it's surprising that Android usage is on the rise and Windows usage is dropping. It's been happening for a while now and you just have to go back into our technological history of computing devices to understand why. When phones were phones and laptops were ridiculously expensive (they still are in some cases), or even before they existed, the one and only way people originally accessed the internet was through a desktop. Windows was certainly more popular than its competitors, i.e. Mac and Linux. It had a massive adoption rate as the one OS that everyone was using. As time went on, phones got replaced with feature phones that had limited internet capabilities and then eventually smartphones came around. This increased the number of Android and iOS users, but didn't significantly impact the number of Windows users. People still needed their computers for stuff. During that time people also began replacing their desktops with laptops because they wanted to be more mobile. Still no real affect on the user base since they were still probably using a laptop with Windows on it. Continuing down that path though, tablets were introduced shortly thereafter and became the new laptop. The average person who just wants to browse the internet, look at Facebook, check their email, and watch cat videos doesn't need an expensive desktop or laptop to do it any more. Especially when they can get something smaller and cheaper like a tablet. If you have a choice between a bulky, heavy, expensive laptop or a slim, lightweight, inexpensive tablet, and they can both accomplish what you want to accomplish, which one would you pick? I think smartphones are what really introduced people to the wider world of mobile computing. They saw what it could do, but it wasn't quite enough to get them to throw their PC's in the trash. It did however give them an introduction to the functionality of a tablet. The tablet is what changed everything. The mass production of cheap tablets (and parents feeling the need to give one to every whiny kid in their house) is what has significantly increased the user base of Android and even iOS. More so Android tablets as they are obviously cheaper than any iPad device. You going to give your kid a cheap $50 knock off tablet of their own running Android or let them use your $800 iPad? Hmmmm...
To be worried, or not to be worried, that is the question. I say not to be worried. Windows will always be around. For content creators and professionals, there are just some things you can't do on a tablet, or that you can't at least do as well on a tablet, as you can on a laptop, desktop, 2-in-1 or what have you. Microsoft recognizes this. That goes for OSx as well. If you notice, the OSx numbers are fairly constant. Most people who have an OSx device probably also have an iPhone or an iPad and use all three for unique tasking, therefore the OSx user base was unaffected by the introduction of smartphones and tablets. I think you will eventually see a point where the Windows user base will plateau and even out as well. What that percentage is though, I don't know. Just depends on how many more people realize they don't need a large, heavy device to watch cat videos on and switch to tablets. And how many more kids they have.
Me personally, I have a desktop. For the longest time it was my only device next to my old Motorola flip phone. At some point I bought an iPod as well. The flip phone eventually got replaced with a smartphone. I wanted to be more mobile so I bought a laptop. It died several years ago and was replaced by an Acer netbook because I realized I needed something smaller and lighter for what I was using the laptop for. I still used my desktop for heavy computing like video and photo editing anyway. The netbook was in turn replaced by a Kindle Fire HD. I figured I still had my desktop so I didn't really need the netbook. The Kindle Fire HD started giving me some issues and I was this close to replacing it with a Samsung Galaxy Tab S2 last year. That was until I put my hands on a Microsoft Surface Pro 4. For mobile computing, it really is one of the best devices on the market, but it ain't cheap like all the other mass produced tablets out there. And no, my whiny kid never gets to touch it. Funny thing is, I haven't had a need to do any video or photo editing as my wife takes care of most of it now. I haven't turned my desktop on since probably last summer. Had I bought the Samsung, there'd be one more Android user on that chart and one less Windows user.