Stroop theorises that the lowering of open-mid to open diphthongs is a phonetically "natural" and inevitable development and that Dutch, after it had diphthongised the long high vowels like German and English, "should" have lowered the diphthongs like German and English as well. In the Dutch equivalent of a wh-question https://puurpositief.nl/navigeren-door-juridische-documenten-de-noodzaak-van-een-beedigd-vertaler-van-nederlands-naar-engels/