I'm not saying that Dermott is better than Weegar, but there are things we need to keep in perspective.
Dermott has 157 NHL games under his belt so far, and has rarely been given the opportunity to move up the lineup. That doesn't mean he isn't capable, it's just where he's been played. When Rielly and Muzzin were injured at the same time last year, Dermott moved to top pairing. He stepped up big time during those games, and wasn't on the ice for goals against, in several of those games, and limited chances... very small sample set though, and he's only 23. Dermott has more to give, than we've seen, and will be cost controlled for some time still.
At the same age, Weegar had 3 NHL games under his belt. At the same age, Dermott was much further ahead in his development. Dermott is physically a bigger man (not by much though), and last year, they had the same number of hits. Mostly playing third pairing situations, Dermott's highest points total, is one point less, than Weegar's career best(playing with Ekblad), in far less ice time.
Last year, Dermott played a ton with Barrie, Holl and Ceci... Weegar played with Ekblad almost 60% of the time.
You put Dermott with Ekblad, and Weegar with Barrie and Ceci, and I think the conversation is somewhat different.
I really like Weegar, and today, he's a better D.. I'd love to have added him to the Leafs. I don't think you give up on 23 year old D men, who has 157 NHL games, and is showing decent promise. At peak... they are probably very similar players.