Same reasons which prevent sane person from being OK with int[] number = [3.14l] should prevent him from being OK with string s = "säд"
No, since this literal can be encoded as utf8 just fine. Keep in mind that literals are nothing else as values written directly into the source. And as is happens your example is a perfect value of type string.
(w|d)string.length returning anything else then the number of underlying code points would be inconsistent to other array types and m aking (d|w)string arrays of code points was a (arguably good) design decision.
That said: nothing prevents you from writing another string type that abstracts from the actual string encoding.
Phobos did it wrong though with handling char[] different from T[].
