The Leafs blueline under Dubas was not "shitty". Your personal feelings do not overrule objective fact.
Muzzin was healthy coming into last season, and was still an important and effective piece, coming off an excellent playoff performance. Permanently losing him to a fluke cervical spine injury that had nothing to do with previous injuries certainly impacted us, but we were still a good defensive team after that. We still got good defensive results. And that's because Dubas had built up a strong defense around him, built up solid defensive ability in his forward group to support, and because contrary to your claims, he did address it in-season, and brought in 3 defensemen (McCabe, Schenn, and Gustafsson), while also bringing in a Selke-caliber forward, and multiple defensively-conscience pieces to boost our forward depth defensive ability and assist our PK.
Again, why would you want Ekholm? You claim we should have purged Muzzin, but you want to bring in and sacrifice other pieces for an equally old and more expensive defenseman with more term experiencing a much bigger decline? It's hypocritical.
We didn't just lose Holl and Schenn. We lost Muzzin to injury in-season, and then we lost Holl, Schenn, Sandin, Gustafsson, Benn, O'Rielly, Kerfoot, Engvall, Acciari, Lafferty, and ZAR. And then we went out and spent big money on some of the worst defensive players in the league, while not replacing the Muzzin loss (or any defensive loss for that matter) or protecting against possible age-related decline in Giordano. Brodie's decline is heavily exaggerated and he was good for us throughout Dubas' tenure. If Treliving thought that Brodie was going to decline this year, he had opportunity to do something about that too. He had positive value.
Dubas took over a bad defensive team that featured defensive cap anchors and a top-4D that would soon start to break down to career-ending injuries in his late 20s, and he was already adding a top-pairing defenseman half a season into his tenure, and had us as a top defensive team within a couple years. Treliving took over a good defensive team and dropped them back to being a bad defensive team, with bad decisions that would obviously have that exact result. He didn't need to fix our defensive results. He just needed to not sabotage it like he has.