In 2018-19, for 6' and taller defensemen who played over 40 games,
96 were drafted in the 2nd round or later (or undrafted).
Contrast that with 23 total defensemen under 6' from all sources.
It's classic confirmation bias every time a tall defense draft pick flames out. There's a low rate of success on acquiring any defenseman outside of the 1st round of the draft of any type.
Putting most of your eggs in the basket that makes up 10 to 12 % of all total defensemen is an error.
There's also a much higher rate of success in finding impact dmen outside of the 1st round than forwards, and especially centers. Order the top NHL dmen by whatever metric you think is telling of their worth, and count how many non 1st rounders are on the 1st page. It will be more than the same exercise with forwards and a lot more than the same exercise with centers.
There's a low success rate for all player types after the 1st round, imo you swing for the fences when you're in a position like ours. The Leafs have lots of good NHL'ers, and probably lots of good ones waiting for their opportunity, but supplemental impact talent is what could really help. If you can replace a Connor Brown, Zach Hyman, Patrick Marleau with someone who's as good as Atkinson/Arvidsson/etc, that's what really puts this team over the top. And the same on the back end, if you can find a Slavin/Parayko/Giordano/Krug with a non-premium pick that's a huge supplement.
Also, the dividing line at 6' is probably skewing your results, the height embellishment principal is probably at its most true when crossing that line from something that starts with a 5 to something that starts with a 6. We saw Dermott turn 6' at 19/20 years old for example