The reason the Canes "lost" the Guentzel deal is that they traded for him to go on a cup run, but they lost in the 2nd round. Even if you want to be disingenuous and argue that Koivunen and Ponomarev are nothing pieces (which Canes fans will tell you is wrong), the point is that they could have targeted bigger needs and/or longer term solutions than trading a lot for Guentzel. This is fundamentally the risk with any rentals, in that you losing means trading for the rental was pointless, but it's especially true with higher priced rentals.
Maybe you can argue that they didn't pay that much in terms of value for Guentzel, but the fact is that they paid that value to lose in the 2nd round and basically threw it away for nothing. The Canes would have made the 2nd round with or without Guentzel and trading what they traded for him didn't move the needle for them whatsoever. They basically gave up Bunting, 3 prospects (one of which is highly regarded by scouts) and a 2nd to not change how their season ended up going.
It's the same way that Shero would lose all of those deadline deals when he'd trade a 2nd and Caputi for Ponikarovsky or when JR lost deadline deals when he'd pay a 2nd for Winnik. Maybe they didn't pay anything that mattered, but they basically threw away assets that didn't tangibly move the needle for their team. With the Guentzel trade, they just gave up a lot for that. The purpose of that trade was to win a cup, not get a nice 15 game run of regular season production.