Although I don't know if you'd really classify these as "old UIs" - they were definitely old, and had a UI. You probably mean "use native platform widgets were possible and any new widgets look as native as possible".
But then some apps bridged this gap nicely. Winamp was very "non native" but the default skin was consistent, clean, and fast.
Yes, but they were exceptions. Interface Hall of Shame has some examples http://hallofshame.gp.co.at/
> Winamp was very "non native" but the default skin was consistent, clean, and fast.
Visual aside, Winamp follows certain standards. Buttons are buttons, for example.
In the 20's, the concern is onboarding users that already have devices and pay money to use software. We don't think as much about familiarity, teaching, etc.
Perhaps the most important thing is to keep the user on the platform, avoiding his departure as much as possible. Thus, a "unique" interface helps with this.
Is it to create a "fresh product" feeling? I don't know.
Old UIs are consistent. Buttons are buttons, labels are labels, and so on.
New UIs are inconsistent. In the facebook interface, the post button is not a button, it's a label. It may have changed since I last used it. It also occurs in native programs.
In older UIs, button size makes sense. Today every app recreates the buttons, with absurd sizes.
Contrast. Designers love low contrast in the new UIs. Here in HN it is sometimes very difficult to read the text.
As a developer, I participated in several meetings about new UIs. "Beauty" is the keyword. The beauty seems to be in the rounded corners and the use of labels instead of buttons. Yes, I created a bunch of shitty UIs.
But most importantly, dark patterns don't exist in old UIs. Dark patterns are the worst thing about the new UIs.