I cannot tell if you are being sarcastic or not? It is a team sport. What does Dubas signing additional players have anything to do with Dermott?
Last season both Rielly and Muzzin were injured for a significant period of time. The leafs lacked any meaningful depth to be able to weather the storm. The Leafs also only had 1 RD in Holl. 2 if you want to count Ceci. Dubas brought in Brodie who is a RD, Bogosian who is a RD and Lehtonen who can play both sides. This gives the Leafs probably their best defense core in several years with depth as whoever would be the "7th" defenseman is very NHL capable and they have at a minimum Sandin and Lily in the pipeline.
A trade for Weegar would be to upgrade Holl, not Dermott. Dermott has top 4 upside in my opinion. He is slotted in on most people's projections because of Rielly and Muzzin. Dermott has almost as many NHL games as Weegar and is 3 years younger. He may or may not end up being better than Weegar but he has potential still given his age. This will be a big year for him to try and take things to the next level.
Rielly-Brodie
Muzzin-Holl
Dermott-Bogosian-Lehtonen (probably Leafs will rotate through each of them to start)