As an architect, I'll say flat design, like other modernist movements, is essential to keep design ornament in check. Left to its own devices, ornament can create delight and texture, but without underlying meaning or intent can quickly become an extraneous taste driven design liability.
Flat design is a rebellion against the inherent subjectivity in design response. There will be many elegantly beautiful examples of its use, but ultimately, the human need for tactile relationships with their environment, and flat design's stress of logic over emotional response will limit its lasting mass appeal.
Flat design's underlying ethos of functional clarity will forever distinguish good design from bad in any field. The best examples of it can be considered high art. However, the fashion appeal towards embracing flat design will inherently lead towards a design homogeny and countless designs that lack any appeal towards emotional response.
But let's be honest, every design language ultimately gets corrupted by fashion.