A 2015 copy of PS CC will almost definitely lack the changes made by Microsoft in the following 3 years. Illegal and end of life software now will stop working randomly as these changes are made and there are no pirates/developers to create new versions.Developers used to moan about this because it sometimes came with a need for new hardware. Because the definition of Windows 10 is always the current one, Microsoft don't have to apologise to anyone when making large, possibly program breaking updates.At least as far as 7 most programs worked on all versions and configurations of Windows provided you had libraries like VC, but unpredictably. No more do developers have to support today's Windows 10 + all the previous versions of 10 he can in case someone has bad internet and turns off updates. If something works on Windows XP SP2 and up, there are easily 25 service packs and a few thousand updates between then and now.

Windows now doesn't have to worry about compatibility too much.Aside from Windows making it harder to update, this has a number of effects.

You do not buy a license to what is on your disk, you buy a rolling license to the 'current' version of Windows on offer. To add a formal answer for anyone else reading this: Please take note of the difference between Windows 10 and Windows 7/8/8.1 as far as what you are buying.