So just by virtue of having cap space, a team get's a free Matt Duchene? I don't think so. That's not how it works.
That's silly logic. Cap space is an asset, it's true. But you can't pretend like every organization could and would have used it the same. It has different value to different teams at different times. Arizona had the cap space, but they didn't get Matt Duchene.
Also, Hall for Larsson wasn't a win for Edmonton because they got a bunch of cap space. They lost the trade because the value they got back was dismal, even when you include the cap space. Trading a good player for nothing isn't a good move, just because you get cap space when they're gone.
You can't tell me that Nashville did a great job trading Subban, because the value they got, even including the cap space, wasn't good enough.