They shipped Pronger out, their only stud D in the 2000s. This is the rebuild after a failed rebuild after another failed rebuild where they literally just swapped Hemsky, Horcoft, and Gagner with Hall, Yakupov, and Pääjärvi which was swapped for McDavid (lucky as heck with that one) and Draisaitl. Notice one thing, not one single defenseman. In a market where free agents dont line up to play you need D that you draft/develop.
Jones
Hedman
Karlsson
Subban
Josi
Carlsson
Rielly
Trouba
All d-man drafted by a team who made decent runs in the playoffs during the Oilers rebuild where they bounced out the first round a couple of times. Unless its a Mcdavid, you build from the back end out. Heck a Slavin would be a lot better the a Nugent Hopkins right now...
They have spent multiple top picks on defencemen
Adam Larsson (basically no.1 overall in 2010 via Taylor Hall)
Oscar Klefbom (1st round, 2011 draft)
Darnell Nurse (no.7 1st round, 2014 draft)
Griffin Reinhart (effectively no.16th, 1st round 2015 draft)
Evan Bouchard (1st round, 10th overall 2018 draft)
All 1st round picks.
On top of that they got Justin Schultz for free which was basically like getting a free 1st rounder.
That isn't the problem. They've spent more on D in the 1st than Pittsburgh + Toronto combined.
If anything I think your theory is wrong, the teams that have done the whole "tank" thing and come out of it have largely used their 1st rounders on elite forwards.
If you're going to do the whole scorched earth rebuild thing you need to come out of it with a multi-line high octane offence, as Pittsburgh, Toronto, Washington are. The teams that have struggled like Edmonton, Buffalo, and Florida and been up and down don't have the high octane offense overall.
If you're gonna go young -- you better be f***ing talented. That seems to be the more operative key.