Well Windows 10 works okay even without an official licence key, it is just the nag prompt on the Control Panel screen and limited options to personalise the machine without jumping through hoops that is different from the full licensed package.
This really is no different to how shareware operated years ago and even today. You tried the software and after a certain time, nagging messages would appear at start up, or certain functions were limited unless you paid for the software. Most people never bothered in my experience. I did pay for a few programs I found useful and used daily, only to find the programs being bought up by large software houses who insisted on being paid again, frequently for every update. Let's just say, I found alternative means of obtaining and running the updated software, that in my opinion and according to the original licence conditions, I had already fully paid for.
Microsoft have what amounts to essentially a monopoly on the software that most computer users see on their screen. The underlying network may run using Unix or whatever, but people have got used to working with Windows in business and in the home. Despite its' quirks and security issues, it works fine most of the time for most people. Considering the complexity of modern operating systems and the variety of equipment that it is required to work on, I think Windows 10 is little short of a miracle in working as well as it does.
Rival operating systems from the likes of Apple rely on strict hardware specifications and strict licensing and testing of add on software to provide security and a pleasant user experience. This does not come cheaply!
Linux, in all its varieties, requires the user to adapt the software to suit their own particular setup. If you hit any snags, you ultimately have to sort any problems out yourself. Nominally free software for everything you might ever want to do with a computer, but a very steep learning curve is required.
I don't think Microsoft have done anything particularly underhand. They are faced with a huge range of different hardware running different versions of Windows. It simply isn't practicable to maintain backwards compatibility between all the versions. Certain security loopholes and limitations have been exposed on the older software that simply cannot be patched. For example, the early encryption methods that were thought to be proof against decryption for a thousand years can now be cracked in seconds using a desktop computer.
One of my maths teachers at school stated unequivocably that there would never be a computer built that could beat a human at chess, because there were too many moves to calculate. It would take until the end of the universe to calculate a move as the game progressed past the book opening stages. Desktop PCs play at a level far exceeding the abilities of most players and dedicated computers have beeaten the world's best human champions. Who knows where quantum computers will take us?
So I think the drive to get as many organisations and individuals as possible to switch to Windows 10 makes sound economic sense. It benefits Microsoft from a business point of view, but it also stimulates a computer industry that has been stagnating a bit because people and companies have been hanging in to outdated software and equipment on the basis of it still works fine, we'll just keep on using it. Microsoft have made it as easy as possible to move to the new operating system legitimately or otherwise without opening the door to software pirates who might supply copies deliberately engineered to leak information. They deserve a lot of credit for that in my opinion.