Design aesthetics follow trends just like in fashion: things are cool for a while, then they’re not, and then they are again—usually after a long period of time in between. Anyone who tells you otherwise is a silly gnome (without the tremendous pointy red hat, of course).
If you’re one of those designers that just does his thing no matter the trend, then more power to you. That’s awesome. And may I be so bold as to say that’s the way it should be.
If you’re one of those designers who surfs the latest trends, then listen up, because if you’re going to start making flat design you better do it well. I don’t want you to ruin it for the rest of us.