They found success too quickly after implementing Nylander, Marner, Matthews. If they bottomed out another 1-2 years they likely would have added a defender.
etc
2017 (Year after Matthews)
Miro Heiskanen (3rd overall)
Makar (4th overall)
2018
Dahlin (1st overall)
Hughes (7th overall)
Outside of 2013 (which was 21st overall) they hit majorly on their 1st round picks with Reilly, Nylander, Marner and Matthews all acquired between 2012-2016. How many rebuilding teams are hitting 4 for 4 on top 10 picks? The problem wasn't that they didn't bottom out another season, the problem was not being able to draft well outside of top 10 picks and not being able to get any major steals in FA or trade.
Colorado was able to get Nichushkin, Toews, Burakovsky and Lehkonen for cheap.
Tampa was able to draft Kucherov, Point, Cirelli, Palat, Killorn, Gourde and Cernak outside of the 1st round.
Outside of Hyman (who had an even bigger breakout with Edmonton) and two solid years of Bunting, the Leafs basically had to pay market rate for their players and didn't have any breakout talent that outperformed their contracts, trade price or draft positions.