Timeless design doesn’t fade as trends go by, but it’s hard to tell if the idea of simplistic “flat” design will be with us for longer than Web 2.0 existed, for example.
As technology advances, we might find that the pendulum might swing back towards the more realistic, or dare I say, skeumorphic, since the interfaces we use will simulate the look and feel of real materials so well that it would make sense in some situations. As long as it’s a natural UI, it shouldn’t bring any dissonance to the user. But…
The ability to quickly design with simplistic shapes rather than agonizingly detailed textures helps with turn around, and allows for more time spent thinking about function and form. From wireframe to final product also takes less effort.
And rather than flat, a better description might vector based design - as another advantage is the incredible scalability of designs based on points rather than pixels. As flat implies no depth, there certainly is a z-axis that can be used to communicate space and hierarchy, amongst other things.
Going forward, the independence from the pixel should be hugely monumental. It won’t be tomorrow, but to build something once, and have it in any shape or form without a degradation in quality can only result in great things — such as more time spent on more important decisions.