Not Windows 10 but Windows Server is a server OS. Usage is probably declining but it's still in use in most companies somewhere for file storage, authentication (AD), databases (SQL Server) or even web servers.
Yep. We are forced to use it because we are a small company with a budget for one real server, and both our accounting and shipping management solutions are Windows-only. I'd wipe it and switch to Linux in a heartbeat if that weren't the case, as the only other functions the server handles are file storage and AD, both of which can be done on Linux.
Maybe not for hosting but I think the Active directory server at the heart of just about every company would disagree with your statement.
SQLServer and Hyper-V are also not to be underestimated.
PostgreSQL and KVM neither.
FreeIPA is a nice FOSS alternative.
You would be surprised about the number of Windows Servers out there. Yes Linux and other Unixes are the winners in the server space but there is a very significant amount of Windows servers in the wild.