I want to know the answer to the following question:
What researcher posited that the lowering of diphthongs is "natural"?
All the background I'm provided with is that "The lowering of the diphthongs has long been current in many Dutch dialects, and is comparable to the English Great Vowel Shift, and the diphthongisation of long high vowels in Modern High German, which centuries earlier reached the state now found in Polder Dutch.". Is that enough to answer the question?
Answer: no