'But in point of fact, this notion of reducing a subject to its “necessary” elements isn’t a universal principle of design. It’s a principle of modernist design.'

Depends on what is meant by "necessary". Baroque art achieves a particular effect by virtue of its ornamentation.

Still, offering a definition of good design that happens to describe minimalism obviously isn't helpful. It's the sort of question begging that just leads to needless arguments.

It's more useful to try to understand how you can have a minimalism site on one hand, and an ornate, busy site on the other, and they can still both look good.