This question already has an answer here:
Working mostly with Linux, I rediscover from time to time Windows and Microsoft's applications such as Internet Explorer/Edge. I also notice that the trend is not limited to Microsoft, although I don't know if other companies mimic Microsoft, or if all those companies, Microsoft included, are following a common trend.
I'm surprised for the past ten years by the evolution in the graphical design which, in turn, has a direct impact on the user experience.
At Windows XP era, a button looked like a button: it had a specific shape with its round corners, a specific background with a gradient—light gray at the top, dark gray at the bottom—and a border. When you clicked on it, the appearance changed to show you that the button is pressed. Similarly, a window looked like a window, icons looked like objects they represent, and so on.
With Windows 10, things are different. No more rounded corners or gradients. All is left are single-color rectangles and circles. A button is no longer a button—you have to guess that the rectangle can be clicked on. Icons are all monochrome now, and in Edge, you can only guess that the rectangles at the top of the browser are tabs. It starts to look a lot like Windows 1.0 and Windows 2.0.
This makes it quite challenging to use those interfaces even for me, an IT professional. Twice, I've seen persons who are unfamiliar with computers, tablets or smartphones, trying to use Windows and Edge, and they were totally lost—“What do you mean click on the button? Oh, I can press a mouse on this rectangle?! I didn't know that!”
I suppose that the choice of Microsoft (and other companies which follow the same shift) to make its products look like they did back in Windows 1.0 is not motivated not by technical considerations (simpler graphics means higher performance). While it could make sense five years ago for low-grade smartphones, I can hardly see how it would make any sense today, especially for desktops, most of which having a powerful GPU.
Therefore, my hypothesis is that this simplification is motivated by factors related to user experience. I remember reading a few years ago a discussion explaining that a bunch of changes in Visual Studio interface, such as a move to monochrome icons, were motivated to enhance the interface for people with disabilities (although I'm not sure how making an icon difficult to understand would make the life easier for those persons).
Is it really a general trend?
Is it related to the user experience, or it has something to do with factors which are completely independent, and the user experience is just suffering from it as a neglected side effect?
If the shift is done on purpose by user experience specialists, what exactly is the purpose? Who benefits from it?