First off, the only comparison here that actually compares to modern UIs is the "functional" architecture.
1) Functional Architecture has blasted off. Every day almost every new constructed or renovated building is dropping the 80s/90s esque design, and getting rid of curves on the outside for sharp edges and trading in the floral wallpaper for straight-forward colors. This is
especially true in larger cities and wealthier areas in the Middle East and basically all of Europe (sans historical areas)
2) Just look at the design of devices. Designs of today do not look anything like they did 10 years ago. Look at the phone in your hand, look at the TV in your living room. Look at the computer or tablet you're typing on. Design has dropped the bubbly, round, soft look of the 90s and embraced a sharp, clean, minimalistic design. Companies have dropped unique shapes for each individual product and have created a more unifying look, each with a unique twist to make it independent. The design language is still universal. This is being reflected in modern UIs to complement.
Dropping the gloss and visual ornamentation does not put the content first, it makes the chrome merge with the content and the controls less crisp. Most content is flat and bright, so chrome should not be flat and white.
And how tasteful textures are more "fake" than horrendous gradients and white spaces?
It's been shown that more modern UIs put the focus on your content, rather than the design itself.
Modern UIs are designed to put content first. The best example of this is Safari. With Safari on iOS 6, you have a static control bar that is constantly interfering with the actual content on the page. it doesn't matter what color the page is, you're always going to be seeing that same ugly candy-gloss bar that can distract you from the actual content. It takes away from what the website designer wanted you to experience.
On iOS 7, the design is translucent and as you scroll and explore the page, it automatically recedes, and the content is pushed forward. You're able to experience what you're trying to look in a more efficient fullscreen manner without any obtrusions.
Look at the lock screens. iOS <6 was extremely busy, dividing up the lock screen into 3 distinct parts that distracted from the wallpaper or the notifications. It took up unneeded space. iOS 7 drops this fragmented design and keeps the lock screen clear and simple, and gives you easier viewing of your wallpaper or any notifications you might have.
Look at non-Apple materials, like Flipboard. Flipboard has been using this minimalistic design since day 1 and it's been doing amazing. Look at the New Google. The pages aren't bogged down with poor design choice; the content you're looking for is up front and center.
Sense 5+ is not very flat and old Nokia UIs were quite flat.
Sense 5 is pretty flat. Also Nokia UIs haven't been very flat at all until around the time the Asha line was introduced and Nokia Belle was launched, which was ~2010.
