I don't think "dated" is the right word for what doesn't work for that dress.
There's a whole heap of brands which do that style of (Amish, hippy dippy, Bloomsbury set, over sized tent) dress for customers who like that style of dress. (me for example) And do it very well.
Phase Eight doesn't do that style of dress. They've tried and they've made a neat , toned down Phase Eight version of it and it doesn't work, either as a Phase Eight dress or a mad Amish dress.