I'm going to nerd out on economics for a minute here to discuss the Carlo deal. Price isn't based on value, price is based on scarcity. Carlo might be the 100th-120th best defenseman in the league, but he's the 30-40th best RD in the league, since 90 (or 75%) of the best 120 d-men in the league all play on the left side. I don't know why so many of the best D play the left side, but this is reality. The supply-demand imbalance pushes the price for RD higher.
So in essence, Carlo as a shutdown RD, is technically more scarce than a LD, who's slightly better than him, resulting in a higher price.
Also, the Leafs have Rielly as a puck mover on the left side, so a shutdown RD like Carlo (even if he doesn't offer much else) is much more valuable to us than another LD who puts up points i.e. a Brady Skjei type. Skjei might be "better" but isn't a RD, and wouldn't offer as much since we need someone on the right side that plays a pure shutdown role to help Rielly, and the contract isn't as good as Carlo.
Carlo is 6'5", cost-controlled for 3 years at only $3.485M, and actually good in front of the net. He's worth the price and I've watched him for 3 games, and he's the perfect partner for Rielly. He just wasn't a fit in Boston to play with Zadorov. This trade is a win-win for both sides, since Bosotn gets a good return for a player that wasn't a fit anymore, and Toronto gets a cost controlled 6'5 RD who's a match made in heaven to play with Rielly during our win now window.