I'm already tired of guessing what is and isn't a button.
Bonus points if you mention the last time a particular trend surfaced.
How could it possibly be anything else?
This is especially true in software. Some additional constraints must be put on to narrow down on a single design. Maybe the constraint is developer familiarity, or developer laziness, or fashion, or whatever.
While there can clearly be 'too much' fashion, its not quite a given that current trends lie in that area, any more than previous trends have.
If you look at clothing design, the fashion side is mostly outlandish and completely impractical. Eventually good ideas filter out of that and get into everyday clothing manufacturing, which is what everyone can actually wear.
The problem with UI design is that UIs need to be used every day, like ordinary clothing, but its becoming like clothing fashion: changing too often and usually full of flashy but unusable cruft. This would be ok if it was limited to a small number of fashionable websites, but instead everyone (but most developers) want the flashy.
For many websites, sure, it is superfluous, but the psychological effects are massive; ignore it at your own peril.
Real applications pretty much ignore all that and look like they always have
The gap from high-end fashion to everyday brands is not so big - take a look at e.g. GQ and look around you to see what people actually wear, and you will quickly start seeing the same patterns and cuts and colours a few months offset (because the fashion magazines tends to report on the upcoming season), just in cheaper imitations - for the most part. And you'll see them spreading as they filter down the cost ladder.
And by the time "everyone" is wearing the cheap imitations, you'll see people in the designer brands wearing different styles and the cycle starts again.
It's the same with UI's: There's a continuous cycle of trying to look modern, refined and exclusive, and so it needs to be regularly reinvented, or it will look old-fashioned, drab and cheap.
It's not about wanting "flashy" (mostly; there are certainly sub-cultures that value flashy above following mainstream fashion). It's about wanting "modern" and "high value".
It depends whether you mean fashion in design principles driven by evidence or visual fashions like in clothing. I think both are inevitable to some degree.
For some reason, we designers are still designing with this mentality.
I can't tell if you're being serious. But if you are, could you explain why?
Otherwise, this is one of many reasons I don't like flat design. It doesn't take advantage of all those pixels. It's just a flat wash of color.
My girlfriend has, I think, an android 2.3 phone. Flat designs are HORRIBLE on it. However, they look crisp on a retina phone, so much that it actually look like the screen is made of physical (material) components.
As for people liking it or not? That's just a trend. We are going back to skeuomorphism in a few years, mark my word.
I think it's best to try to get rid of unnecessary garbage in your UI, maybe that's the skeuomorphism, or maybe it's the amount buttons, images, content, etc. If done well, flat design is a usability win.
Flat itself is a design fad like any other, and the reality is the app's appearance and adherence to trends is a real signal of quality to most users.
There's a difference between having a platform-specific GUI/style-guide (which I support), and changing it every few years for no apparent reason (I understand e.g. the change from Apple OS 9 to OS X - they were simply catching up with the hardware improvements - but OS X to OS Y(osemite) seems quite arbitrary).
I think it's always been fashionable to have a modern UI. Just look at windows (the concept). I think the trends are just moving faster nowadays, as the tools to related to developing UIs become more accessible.
Imo flat design came largely from responsive web design. It's just easier to build nice looking ui which adapts to any screen size when it's flat. This of course was why it suited android, because they have hundreds of diff screen sizes to cater for...