Within the last 30 years the leafs at certain times, when they were more competitive than usual always had a above average defensive as a unit/group, for example the burns era with rouse/macoun/Lefebvre/ellett, then a few seasons later during the Quinn era, the leafs had a decent group of defensemen such as yuskevich/Markov/kaberle/leetch/svehla/McCabe. Even now in the recent era, the leafs had D like muzzin, Brodie , McCabe, tanev, giordano and rielly.
But decent doesn’t mean elite, a defenseman who is the best one of his team doesn’t mean he will be the best D of another team. There is no automatic equivalency, historically the best and most competent Dmen of the leafs would have been relegated to the 2nd pairing at best on legitimately elite championship squads like the 2000 devils (stevens/niedermayer/Rafalski), 2001 Avalanche( Bourque, Foote, Blake) 2002 Red Wings ( chelios, lidstrom), 2007 Ducks (pronger and niedermayer), 2013 blackhawks (Keith, seabrook ), or 2020-21 lightning (hedman, mcdonagh )
and it’s true that they never had a true pronger/lidstrom/niedermayer/hedman caliber defenseman on their roster. A blueliner who clearly belongs in the elite tier of defensemen across the league without question. Unfortunately these ones are extremely hard to find in the draft, they’re impossible to acquire through trade and they rarely become unrestricted free agents in their prime years because teams who have these kind of defensemen will give them lucrative longterm contracts beforehand to keep them on their teams as long as possible. These are the true reasons why any team, let alone the maple leafs struggles to obtain them.