Umlauts are even rarer in English than you suggest: the diacritical marks in "naïve" and "coöperation" are diaereses. I think this just makes your overall point stronger.
Re: “Ünrest”
Umlauts are even rarer in English than you suggest: the diacritical marks in "naïve" and "coöperation" are diaereses. I think this just makes your overall point stronger.