Up until the Windows 10 free cut-off date, Windows forcefully installed itself onto some people's computers without asking. Sometimes with Windows Update turned off. Not all of the code was copied form a central server, actually any computer already with Windows 10 can be requested to send parts of the main install, or updates. Some people complained that it didn't give any preference to computers already on the internal network, I think this has been fixed in the Anniversary Update. Copying itself confirmed!
Has a detrimental effect. Windows 10 removes some features automatically like Media Center, Windows DVD Maker, DVD playing, Desktop Gadgets, Start Menu, Solitaire, Minesweeper, and Hearts. It also removes some third party software as well, or requires you to uninstall and re-install it. Some games and programs just no longer work at all. Now you can get some of these things back with third party programs, or hackers getting Solitaire, Minesweeper, and Hearts to work again, but it is the same thing with a virus, or malware. Windows 10 is both a malware and virus confirmed!
Oh, but I'm not done. "Windows 10 does not destroy or corrupt data unless Microsoft deems that it is for the user's own good." Microsoft now supports FLAC files natively, well kind of, it will read and play them until for some reason it decides to corrupt the files so that it, nor any third party software can play them anymore. Microsoft is aware of the problem, but I have not seen a fix for it, even though it has been a problem since the first release of the OS.
As for corrupting data, a common problem with Windows 10 is that it will sometimes corrupt core OS files such as the taskbar, start menu and windows update. Common problem! I have even experienced this myself. It is so common, they actually have a specific easy fix baked into the OS to solve this. The only problem being that the easy fix uses windows update, so if that is hosed, you can still fix the problem, but it is really complicated. I ended up doing a refresh instead. Aka, Windows 10 corrupts the OS making it a virus.
I'm sure some people with ASUS motherboards can tell you about their hosed systems from a Windows 10 update
How about the Anniversary Update that breaks all USB2 web cams making some completely inoperable?
As you point out, it gathers sensitive information and displays unwanted advertising in their replacement for the old games, in the start menu and in notifications. Making Windows 10 definitely malware.
As you can see, not only is Windows 10 malware, but it also meets every definition of a virus as well.