DarrkPhoenix: Keep in mind as well that as this survey was optional it only reflects the statistics among those that chose to engage in it. This is important because if there are any correlations between people who chose to take the survey (vs not take it) and one of the survey options it can dramatically skew the results. For instance, if there were a correlation between people who chose to not upgrade to Win10 and people who tend to choose not to unnecessarily hand over information to companies then the non-Win10 results could easily be under-represented. No idea if such correlations exist, but they're important to keep in mind when considering survey methodology.
Indeed, for example it could be possible that people with the mentality to not upgrade their OS to Windows 10, might also have the mentality to not do hardware surveys. Myself for example.
Mind you, I am not fundamentally opposed to the Steam hardware survey's primary data focus and even went and filled it out once - but, then I found that part of the hardware survey scanned my computer for all software applications and was going to include the list of all installed applications etc. in the data packet sent to Steam. Now that - that is something I vehemently object to. There's no good reason for Valve to mandatory require that information, and so - I hit cancel and I do not participate in the hardware surveys. The software I have installed on my computer and other data on my hard disk etc. is NOT "hardware".
So, people such as myself who are aware of this are going to end up unrepresented. I surmise that people such as myself that have a higher concern for privacy related issues are just as likely to avoid upgrading to Windows 10 for the same reason. There's no way to measure us, but we're out here. Even if we're a small group (and we probably are in all honesty), there are likely many other pockets of users that fall into other categories which aren't counted as well.
Still, even though that may be the case - companies can only try to do their best to gather the data and then act upon the numbers within a degree of error margin, and Steam stats are probably the best information out there on gamer hardware availability even with errors in the counts.