It is about Microsoft being a neutral player in a multiple platform world. Microsoft want people using their tools as it means they are more likely to use their platforms and services. Be it PowerShell on Linux or Bash on Windows or VSCode on macOS they want to make the best tools in the hope that it will push companies/developers towards Azure/SQL/Cloud as that is where the future for MS is.
Windows is on life support and they know this. The future isn't about companies buying hundreds to thousands of Windows licenses but an instance of an OS which is billed per X/month. They have seen businesses are more than happy to enter into such a model with Office 365 and The Cloud so it makes sense for them to move in that direction.
How is Windows "on life support"? Desktop/laptop computers are not going anywhere, at least until they invent some kind of VR (and no, mobile OSes like iOS are not suitable for doing real work and managing data in a networked environment the way desktop OSes are). Windows still has at least 90% of the OS market. People and businesses could change to an alternative, but they just aren't, and they aren't going to unless something really drastic happens to force them to (and this would likely have to include many popular applications also providing support for one or more of the alternatives). In the business/enterprise world at least, there is simply no indication that Windows is in any danger at all of losing marketshare or revenue, in fact it's probably going to increase since businesses seem to love the software-as-a-service model.
MS could probably just make Windows free for home users (and make even money on them with advertising and selling telemetry data and also support calls), maybe small business users too, and then soak the big businesses with high fees for their site licenses and SaaS and other services.
Windows revenue has been shrinking for a few years now. The growth is in Azure and related cloud technologies.
Sure Windows probably won't ever go away (until a massive paradigm shift as you mention) but businesses are not upgrading like they once were. You have a mixture of reasons for this, BYOD has chipped away at some of it and forced companies to allow non-Microsoft (i.e. Apple) systems to connect to the network for remote and onsite.
With more and more work being done in the browser it makes specialised software that forces upgrades to Windows out of the picture.
I have worked for a number of large companies and recently the reduction of annual spend on Windows clients is pretty staggering. With almost all "managers" going BYOD with their (mostly) MacBook Pro's and onsite machines lasting a good few years longer than they used to it means we just don't need to buy as much from Microsoft.
You can see Microsoft know whats up as well. Windows Enterprise subscriptions and Azure are the future for Windows in business. For home users Windows is already "dead" from a revenue perspective with the exception of charging via the OEM at first sale. Android, macOS, iOS, tvOS, all the operating systems home users are aware of are free upgrades on supported devices. This is where Microsoft has a problem as obviously they support everything so they can't lock out some users with a "2011 Dell XPS". However going forward I expect we will see them do something along these lines with limitations around what minimum hardware is supported so you will only get upgraded to the latest Windows if you have AVX2 or such based hardware.
I would agree that Windows is nowhere near being on life support but I do think that the size of the desktop OS market is shrinking and that in the future we may see a lot more home computer users move to tablet only or something like a Chromebook.
Enterprise users are much more slow to move but as enterprise SaaS offerings become more robust we may see a lot more businesses move towards a thin-client (a la ChromeOS) and SaaS model to meet their needs. Microsoft is doing the right thing by quickly getting out in front with Office 365. If they hadn't maybe something like Quip or Google Docs could have starting stealing significant market shared but now I don't think many enterprises would bother switching if they can stay with Microsoft.
Windows is on life support and they know this. The future isn't about companies buying hundreds to thousands of Windows licenses but an instance of an OS which is billed per X/month. They have seen businesses are more than happy to enter into such a model with Office 365 and The Cloud so it makes sense for them to move in that direction.