The main reason it seemed to be so short was because for a while, they were accidentally rebuilding while trying to win. Morgan Rielly and William Nylander were added after years in which the team was bad because it was poorly managed and assembled, not because the team was actively trying to rebuild.
Even Mitch Marner was drafted after Brenden Shanahan’s Year of Observation. (The first season after he was hired, he kept Nonis and Carlyle / Horacek and the team was in a playoff spot near Christmas before they suddenly couldn’t score and lost enough games that Carlyle was fired).