The return itself wasn't bad in a vacuum. An average 2C for a #3D is pretty fair value. The problem is the Leafs were on a food stamp budget and Dubas went out and bought booze. The last thing they needed was another soft, defensive liability.
I also think there's a lot of revisionist history here around Kadri. They see the PPG player that won a cup and think that's what the Leafs gave up. The thing is, that was one season when coincidently he needed to get paid. People forget about the countless suspensions leaving the team with $4.5m that could've been spent on someone who could play in the playoffs. They forget he was a top 10 pick who couldn't even unseat Tyler Bozak as 1C on some awful Leafs teams. And they don't see the overpaid liability he's been in Calgary.
The funniest part about all this is Kadri declining a trade to Calgary because he was too good for them, only for him to come crawling back there when they were the only team stupid enough to pay him. I can't believe I'm defending Dubas here, but had Brodie been a Leaf a year or two sooner in this trade, it doesn't look bad at all. But Kadri had to f*** over the Leafs one last time on his way out the door.