Please send full bottles of 1945 d’Yquem or older to me and I will have them analysed and answer the question.
Whole lotta’ hand-waving going on here. I’m a chemist, and I have no idea. And I just tasted through a bunch of Climens wines tonight, ranging from 66 to 88, and they all seemed about equally sweet to me, taking into account vintage variation. In my own experience, “good” bottles of old Port and Sauternes, and other sweet wines don’t lose much of their sweetness with age, though obviously they change. My hunch is that more often than not, wines that have changed significantly are due to oxidation from imperfect cork seal.
PhD organic chemist who sits next to me here at work said polymerization is the right path for the perceptible sugars to reduce over time. After that it all just sounded like buzzing in my ears.