First, PMV made a sensible observation. They are compelled to change the appearance of Win 10 over 7/8 because they have to promote it to mainstreamers, or even "new" computer users. None of those folks would be much interested in the new OS if it just "looked like" win 7.
Sorry, but I don't buy this at all.
1) I would be willing to bet that the number of users who upgrade because of a new version of Windows (no matter how much the eye candy has been changed) is probably a single-digit percentage. Most people upgrade these days because their computer is creaking for one reason or another.
2) While I will acknowledge that if something looks new then it will attract more interest, however for example OS X's look has barely changed in comparison in what, 15 years? Furthermore, attracting interest isn't necessarily a good thing. The reason why movie sequels sell well is because they're generally serving up what is familiar to the audience. However in computer usage terms we're not talking about a two-hour experience, we're talking about something that will rack up hundreds or thousands of hours of experience, something they have to use (or consider to be indispensable). The only people who are interested in changing what is familiar to them in this context (UI) are those who are interested in the change itself, e.g. the kind of people who frequent this forum. When most people want something new from their UI, they change the desktop wallpaper, not re-arrange all the icons, taskbar and Start menu entries.
The only visually obvious changes (for the sake of change / "newness") I would make is the Windows splash screen on each release, the default wallpaper, and the addition of some new app. Otherwise the only UI changes that would be allowed by general development policy are ones that are considered to be necessary in order to make an actual improvement (e.g. feature addition / workflow improvement / fixing what is generally considered to be a borked UI by most accounts).
Furthermore, Windows XP and Windows 7 are generally acknowledged as the most popular versions of Windows. IMO, here are the reasons:
1 - Many people were upgrading to XP from 9x (ie. they didn't see Win2k), so there was the massive reliability / performance improvement. It had a default Fisher Price UI which maybe 1% of people actually liked, but it did the job; once you got used to it, it was OK, or you could switch to the classic UI without any disadvantages.
2 - XP stayed around for ages. People got comfortable and familiar with it. They probably changed computers at least once during XP's era, and they didn't have to worry about whether stuff had been changed between the old and the new computer. XP's lingering market share was a testament to this.
3 - Win7 was very similar in looks to Vista (I can think of two differences off the top of my head, the look of the taskbar and the way that sub-sections of the Control Panel were laid out). However, it fixed the performance problems of Vista and performed much more like XP from a cold boot. Most of the changes to Win7 were under-the-hood yet that didn't stop it from having having the dominating and lingering market share in a similar way to XP.
As far as I'm concerned, the burning question is will MS manage to resist re-arranging the deckchairs of Windows 10 every year or so, or canning some random app along the way: MS has a horrible record of introducing an app only to remove it a few years later in a new version (or in the case of e-mail, completely replacing it or completely removing it depending on which way the window blows!).