Re: IMHO, It would be fair
"IMHO, It would be fair to compare the shares of each OS actually installed by users themselves"
No, that would be deliberately biasing results in favour of the result you want to see. It would also be asinine since uninstalling OSX on a Mac is an incredibly niche thing to do, and people don't uninstall Windows to install a new Windows (you said you wanted to measure people removing pre-installed OS). Probably 99.9% of manual Windows installs are done on machines which came with Windows in the first place.
If we purely focused on % of installs regardless of previous OS, then you'd still see OSX users upgrading to newer versions of OSX but the vast majority would STILL be Windows installs.
And that's even leaving aside the elephant in the room which is that most people are happy with the default OS. Saying anyone who doesn't re-install their OS isn't a relevant data-point is just stupid.