The argument isn't how people will eventually go to 10. If Microsoft stays on it's plan, everybody will be on 10 at some point (the last OS). The current argument is speed, which gauges how well their salesmanship on the new OS is working out. At least we can say it's doing better than 8.
Problem is times change from year to year even from month to month, there are more tablets,smart phones out there now then last year and year before, so comparing to previous Operating Systems is not without errors.
Fact is Microsoft Operating Systems are up against tougher competition now, so comparing sales to previous Microsoft Operating Systems is not really valid.
How many sales have Microsoft lost due to competition from Android, Linux, Apple etc, now that would be a better gauge, however hard to do in the real world, yes would be great if they did that for the last ten years up to the present day, but that won't happen.
Remember those users out there that ditch Microsoft based PCs for Android/Apple tablets or smart phones, yes I know a few that have done so, change of the times .
So point being does it really matter?..Obviously not since you either get with Win10 or end up on older Microsoft Operating Systems that will be phased out well before EOL of 10.