I don't share the hate for flat design.
It's cleaner than the others, simpler and less distracting. Easier on the eyes, too. It takes itself seriously and does so successfully imo (nice try, aero). It feels professional in a way all the previous eras don't - they seem almost child-like by comparison.

Modern design cultivates recognizable interactions by following conventions and common design language instead of goofy icons and high contrast colors. To me, modern software interfaces look like tools; the further you go back in time, the more they look like toys.

Old designs can be charming if executed well and in the right context. But I'm glad most things don't look like they did 30 years ago.

I'm guessing many people associate older designs with the era they belonged to and the internet culture at the time. Perhaps rosy memories of younger days. Contrasting that with the overbearing corporate atmosphere of today and a general sense of a lack of authenticity in digital spaces everywhere, it's not unreasonable to see flat design as sterile and soulless. But to me it just looks sleek and efficient.
I used to spend hours trying to customize UIs to my liking, nowadays pretty much everything just looks good out of the box.

The one major gripe I have is with the tendency of modern designs to hide interactions behind deeply nested menu hopping. That one feels like an over-correction from the excessively cluttered menus of the past.
That and the fact that there's way too many "settings" sections and you can never figure out which one has the thing you're looking for.

P S. The picture did flat design dirty by putting it on white background - we're living in the era of dark mode!