Apologies if I misunderstand, but your "reverse nesting" syntax already works with scss. I.e. `.a{ .b & {}}` => `.b .a {}` ... I'd quite honestly use it fairly often, usually with state/data modifiers on an ancestor (or ancestor sibling!)
I assume that for the same reason that they cannot support direct nesting with no extra tokens (parsing issues), they also cannot support the simple SCSS version and instead went for the "@nest .b & {" mouthful.