I think what you're observing but misdiagnosing is that you can occasionally get a team of mediocre defensive individuals to play great team defense, but you can't get a team of mediocre offensive individuals to play great team offense.
So Damian Lillard or Kevin Love or James Harden can be "hidden" on D. But s guy who can't shoot, dribble, or pass completely detonates your team's ability to score points. There is a higher premium on transcendent offensive players rather than transcendent defensive players, because transcendent players can have make much more acute difference on that end of the floor.
With all of that said, there are players on the defensive side of the ball who raise their entire team's defensive level. We saw that with KG; his 2007 championship season you could conceivably say he was the MVP of the entire league, even though he wasn't even the best offensive player on his team.
He just elevated the whole team's defensive effort, intensity, communication, etc.
Russell was (from what I've heard) that way. He was the ultimate team player, a guy whose defensive effort made the team and gave it its identity.
So yes, trading one of the best defensive guards in the league (like Smart) for one of the best offensive ones (like Lillard). Is a no brainer. But don't be too quick to minimize the influence of these types of players. They are franchising changing in terms of defensive identity.