Never? 1) Users didn't ask for flat design 2) There's absolutely no evidence that flat design are better and there are evidence that they are worse Yet I don't

Never? 1) Users didn't ask for flat design 2) There's absolutely no evidence that flat design are better and there are evidence that they are worse Yet I don't expect that UI will go back to 'classic UI', many of those who create UI care more about being perceived as modern (or more accurately not being perceived as outdated) than efficiency, so flat design will probably be replaced by another 'fashion driven design'..