I think Windows 8 was pretty dumb as well but that doesn't mean I think flat UI or a modern UI in general is a bad idea. Of course we all have preference, but to me, saying we should go back 10 years in software design is like saying we should go back 10 years with hardware performance, car components, science research, education system, government laws, health and safety guidelines, etc.
You can't just "go back". The entire world is constantly surrounded by new smart phones and computer systems, the term here is "ubiquitous computing". All the companies want to get ahead of the game in design for appearance and user experience to get an extra leverage on making more sales and keeping existing customers.
No matter what design you have, you can never please everyone. There will always be people who prefer the previous or want something better. That is just something that happens in life. I want a Ferrari but I know it isn't going to happen. That doesn't mean all Ferrari's are stupid and should be banned from being driven by people who do happen to be able to afford them.
It is what it is, and in reality it is just preference. It would be unrealistic for me to tell someone "All UI styles apart from Flat UI are stupid because I prefer Flat UI and Flat UI is the best for the entire world because of this" because it is just a preference which I've adapted to and enjoy having while it lasts. At the same time, saying that any UI other that Flat UI is stupid would just be an opinion as well, a preference of preferring another style of artwork and taking a disliking to the one in question. None of it is factual.
What is factual though is that certain styles are applied for certain environments, and/or the trend demand. That isn't about whether a design is good or not, but it's business. Companies are constantly trying to make changes to find ways to improve user experience, make sure colours are matching up well, brainstorm and implement new ideas which they believe is innovative. They can't plan a UI for ages and expect everyone to like it. They can work hard, take risks and hope for the best. What if it doesn't work out? Keep trying, or scrap it and try something else.
Look at Avast. They started trying with ideas like this.
Then they started trying things like this.
Now they are trying things like this.
There's a huge difference. Notice how the Status tab is pretty much empty? Maybe they thought it would be easier for it to be this way, so it truly is simple and minimal, nothing confusing. Just straight to the point of whether you're protected or not by default. One more click and you are at the Protection settings, or one click away from all the Settings. Whether it is a good or bad UI is down to you, but my opinion is that what they are trying is maybe a step forward.
Now let's take a look at Windows XP.
As
@Umbra said, Windows isn't just for Desktops anymore. If you look at the XP UI in general, things tend to be much smaller by default like the task bar, start button, etc.
Now let's look at Windows 10.
Now if you ask me, the Windows 10 UI looks much more adapted for a cross-device functionality. You could say that Microsoft could do a whole separate UI for mobile, but then we are back to user experience and not just appearance of styling! If you have customers using Windows Mobile and a PC with Windows 10, it is likely going to be a lot easier and hassle-free for them to have a similar experience across both devices... Thus they don't have to re-learn more for each separate device, they'll be used to the feel and usability. Notice how the UI components tend to appear bigger? Less perfection with moving the mouse, quicker actions.
This isn't me saying one UI is "better" than the other because of my preference, I am just trying to explain that there is so much more to it than what meets the eye. So many factors come into play.... Your target market, the general trend people are statistically liking, what other companies are being successful with, ease of use, performance on resources with animations, etc.
When I was brought up, we had a PS1 or PS2, Nintendo DS and a PSP 3000. Now we have a Nintendo Switch and a PS Vita, or a PlayStation 4. Why? Demand! Target markets! An teenager probably wants to play an Call of Duty game nowadays, not a 3+ Mario Kart game on a 10 year old Nintendo DS. That doesn't mean the old Nintendo DS was awful nor the game, it is just how things have changed. The Nintendo Switch being a DS replacement for younger children, the Xbox One/PS4/Gaming PCs being a replacement for teenagers, while still having features for younger players. It's all part of the designing world and how we as humans change over generations.
Designing is a true work of art, a masterpiece. It cannot be rushed and it can never be fully completed, because as said, you can never please everyone. If we stick to the same thing forever, it is like never trying to figure out the cure to life threatening diseases, and just sit assuming the world will solve itself out for us. That is no way to live in my opinion... We need to experience new things by progressing, using our improved resources to our ability to try and innovate as much as possible.
You never know what is around the corner, never ever count your chickens. Life is life, designing is designing. Everyone is different. We all have different minds and imagery. This is why designing is so exciting, because you never know how people will react. A majority may love or hate it... Trial and error, you live and learn.
Take it how you want to take it, use whatever UI you want. But the way I see it, there's so much more to this. Change can be good sometimes. We can't find something more convenient for specific purposes if we don't attempt to! E.g. I am sure a pen eraser is convenient for someone who prefers using a pen but is in fear of mistakes, now they don't have to use a pencil if they hate them.
