I'm a little confused by your arguments.
You are comparing a balanced line in Boston with Pastrnak (a winger) to the importance of a defenseman? Yes, pairings matter, but when you play DEFENSE, you are relied on to play DEFENSE. There is a reason defensemen are more highly valued than wingers. If Boston calls Carolina and offers Pastrnak for Slavin, do you think Carolina takes that?
As for Barrie's partners, Holden is still playing a viable role with Vegas and Nemeth is in Detroit. Barrie's play has Barrie'd his partners time and again, and to think that Graves would not have fallen subject to that is laughable.
You argue Barrie is a dynamic puck moving defenseman, but he is bad at outlet passes. Wouldn't passing to other offensive players to create scoring chances be considered dynamic. However, if you suck at passing and can only create offense by rushing the puck, wouldn't that make you one dimensional or not dynamic?
Again, I do not care about anything in Toronto. Toronto made a bad trade, not because Barrie is awful, but because he was not the correct fit for that team. This is the drum I have been banging for years when it comes to paying a player like Barrie. He has to be put in the right system to succeed and it is a very specific system. Otherwise, you are going to get caught with more of a liability than a commodity.
You and I will never agree on any of this though. Fact is, you think that offense is the best defense, and I believe that true defense wins championships. I think that Colorado is having more success this year because the defensive side of their game has vastly improved and that has benefited the offense greatly. Agree to disagree.
Does Boston take that deal? Probably not. Pastrnak, despite his one-dimensional game, is a near-elite if not elite scorer. Plus he's locked in for a reasonable sum until 2023. Carolina might think about that too as Svech and Aho setting up Pastrnak might be fun to watch, but I don't think they part with their best all-around defenseman. And Slavin ain't just some stay-at-home guy. He has a very well-rounded game.
I know what a defenseman is. Shouting DEFENSE makes me think you're still in the 90s where positions and roles were insanely rigid. There is much, much more fluidity in the game than there used to be. And yes, a good, aggressive offense is the best defense. If you think sitting back and blocking shots and relying on goaltending is the way to go, please see how that's working out for the very defensive-minded team the Avs are currently playing.
Holden is a third pairing fringe defender whose defense is uproariously bad. Vegas is elite defensively though, so they can roll him out without consequence. The Avs used him in a top-four role and he got exposed CONSTANTLY. I don't mean to constantly slag the guy as I think he's a good guy, but he was an NHL journeyman for most of his career for a reason. Nemeth played a solid role...on the worst team in the NHL by a country mile. Not entirely his fault of course, but your argument there doesn't really hold up. Barrie never had great defensive partners in Colorado. I'd argue Nemeth was probably the best (definitely in year two when he finally stopped turning into an icing call factory). To say otherwise would be a lie. So I don't think Graves would look as good as he's looked alongside Makar, but it ain't like Barrie didn't have the tall task of making a mediocre or bad defensive partner look a whole lot better.
The only thing I'm taking exception to in your arguments is that you're greatly oversimplifying some things. The Avs did not magically become the 7th-best defensive team in hockey because they got rid of Tyson Barrie. That helped, yes, but they made some key additions up front defensively and addressed scoring depth issues. Now they spend a whole lot more time in the attacking zone when before, even just a year ago, they had a lot of one-and-done possessions. Keeping it in the other guy's end of the ice is by far the best form of defense, because the opposition can't do anything with it if they don't have the puck. It's better than just traditional defense, which the Avs are also much better at than they used to be. They can get the puck away from opposing players and move it out of the zone better than nearly every other team in the NHL. That's effective defense.
Yeah...defense wins championships. We agree on that at least. I'm grateful the Avs employ a form of defense that isn't a crime against hockey, unlike the Flyers, Coyotes, and to a certain extent, the Blue Jackets.
FTR I'm not sitting here saying the Leafs failed 100% because of Babcock and shaky goaltending. Those were two huge factors, but truth be told, the Leafs were doomed from the get-go. The "Shanaplan" tried to bridge the gap between old school and new school but the men involved wanted no part of that. Dubas is by no means free of blame, but he's but one factor of many.