I’m glad they were. A few months ago, I visited the Metropolitan Museum of Art’s now-closed Michelangelo: Divine Draughtsman & Designer, an exhibition of Michelangelo’s drawings. This great show wouldn’t have been possible if Michelangelo had burned all of his drawings.
According to Ingrid Rowland and Noah Charney’s The Collector of Lives: Giorgio Vasari and the Invention of Art (my source for this entire fun fact), Michelangelo ran into some problems when he eventually abandoned his preparations. Looking at the partially-carved remains of some sculptures Michelangelo left unfinished at his death, they have observed that the great sculptor didn’t leave enough stone to fit the whole rest of the uncarved body. Oops!
Source: Rowland, Ingrid and Noah Charney. The Collector of Lives: Giorgio Vasari and the Invention of Art. New York & London: W. W. Norton & Company, 2017.