I know your criticism is about flat design but really, I think the widespread use of of any boilerplate/framework contributes more to stale design than concepts like "flat".
In my defense: I tend to see the established frameworks more as best practice design patterns than odious threats to originality.
Maybe that's just a rationalization but we need to work on what's come before us, what's been built us for us. We can't still be running round with piles of punch-cards if you get my meaning.
The work of our predecessors in both science and technology is a valuable gift and the digitisation thereof has ensured that this gift will constantly accumulate and be available (as long as good humans will risk their own liberty for a principled stance on digital freedom) so there is a point at which we take the title 'developer' literally and literally develop further on what has been achieved so far.
Innovation and originality are precious things and we should certainly value their rarity and value but you don't just stop at the wheel. You build a car.
It is very important to be thoughtful but we must also be cunning with our directions.
Focusing releases of energy on variations of the 'top menu; hero section; columns of content' layout might be less efficient than focusing on how better to interface to these things via those patterns, solutions that are transferable between contexts, adding more affordance across devices, etc.