Do you have any evidence that it's a large proportion of the user base? Windows 10 already makes up the majority of PC usage share in the USA and the UK (on Statcounter), and Microsoft says corporate adoption is at unprecedented levels for a new version of Windows. (Mostly pilots at this stage, obviously.)

More people use Window 10 than have used all versions of Mac OS going back to 1984, and in its first month, it had more users than Linux got after 20-odd years.

There are always hold-outs: I gather some people still cling to OS/2 and maybe even Amigas. There used to be a cult that said you'd have to prize XP out of their cold dead hands.

However, the fact is that Windows 10 absolutely has to work for the US Defense Department (which is already switching to Windows 10), Bank of America and practically all the S&P500, so you can be confident that it will.

Hasn't that always been Microsoft's modus operandi? They could have cracked down on pirated versions of their software (Windows, Office), but they didn't because they knew that piracy helped in making their software the de-facto standard.