Ok, let's sort this out:
Heavy Metal or Hard Rock, in a seventies and eighties sense at least, requires/d a vocalist with high pitched vocals to sing above the guitar riffs which would not actually go down in volume and density during the verses in many cases. Unless you produce music like the old Billy Idol stuff was produced with the vocals really loud and the verse riffs subdued, a less high voice has issues cutting through, especially live.
This Loudness chap doesn't sound to me anymore screechy than Brian Johnson, Geddy Lee, Paul Stanley (especially in the eighties), Vince Neil, Robert Halford, Udo Dirkschneider etc, in fact he sounds less screechy! Vince Neil is a prime example: He neither has a nice voice nor is he a particularly accurate or variable singer, but that one screech tone he does gets him heard no matter how much the other three play.
And of course: The higher the vocalist sings the less loud he has to be in the mix and the less his vocals take away attention from the instrumentalists. When Ian Gillan wanted his voice up higher in the mix on DP albums, his then guitarist (name graciously avoided!) snarled: "Who do you think you are, f***ing Tom Jones?!!!".
Confinements of the genre or why Led Zeppelin wouldn't have worked with Ian Hunter or Jim Morrison as vocalists though it would have done their lyrics a hell of a lot of good.
And much as I love Iggy and especially this song, listen to how loud he is in the mix for his voice to be actually heard above the ostinato riff (and compare it to the little Japanese guy yelling against the Loudness riffs at much lower volume!).