The whole point of ligatures is that they are visually subtle enough that you don't consciously notice them, while removing distracting collisions of adjacent glyphs. The German rule seems entirely pointless and counterproductive.
That said, OP is wrong that ligatures would "break Unicode". The only ligatures that are directly encoded exist for historical reasons. Fonts can and should make contextual alternatives, and this is a normal part of typesetting for several languages.
Unicode breaks Unicode, it's not meant to be consistent, only compatible.
> The German rule seems entirely pointless and counterproductive.
The rule provides visual hints for how to pronounce words. I think that is way more useful than slightly increasing the readability by joining glyphs without context.
That said, OP is wrong that ligatures would "break Unicode". The only ligatures that are directly encoded exist for historical reasons. Fonts can and should make contextual alternatives, and this is a normal part of typesetting for several languages.
Unicode breaks Unicode, it's not meant to be consistent, only compatible.