> if we need @nest some times, why not always make it required, for consistency

Prefixing a parent selector seems super ugly. The facility is available in SCSS, but I suspect it is not used much (e.g. comment above not knowing the feature existed).

Using & for the usual case of appending selectors seems sensible to me.

Aside: I suspect using & to avoid lookahead parsing is important for efficiency and to avoid DoSing the browser (especially malevolently).