I don't think flat design is the issue (although I agree that it's getting to be as overdone as skeuomorphism back in the early '00s). The real issue is that it's discarded all of our carefully built up visual design language (ie. what makes things feel 'intuitive') in favour of a churning mishmash of beautiful new impediments to understanding. The disappearance of top menus. Inscrutable icons instead of text. Hamburger menus, arcane X-finger-slide-from-Y-edge style secret handshakes, it just keeps getting worse.