They don't need to be leaf components to still have this strength. I've worked with design systems which have a <my-layout> component near the root which defines sidebar, header etc slots. This still works nicely with react, Vue etc
But I agree they tend to be better suited as "leafier" components
Honestly I couldn't care less what the original proposal said! Things evolve. Utility is found in unforeseen places. That's the nature of ~everything. Personally I'm not a fan of that kind of component but not aligning with some original, preordained vision is a minor nit
But I agree they tend to be better suited as "leafier" components