But software rarely exists in a (design) vacuum. It runs on Windows, it runs in a browser, on an iPhone… or maybe the software already exists and it's just a new feature.

That's why "we live in a world of Frankenstein UIs". We don't need "utopic designs" but rather consistency. Both in visual language and interaction design.

There are a few notable exceptions like HMIs on industrial machines or a car's entertainment system.

Of course, incorrectly implemented design exists, too. No doubt about that.