Unpopular opinion (and long post) alert regarding the argument that we need to swap a small decline in defense for a big upgrade in offense:
I think the Conley deal may have already been that move. I watched two or three full games of Conley this season, specifically trying to focus on his offensive and defensive strengths. Defensively, I think the Jazz have taken a step backward from Rubio--hopefully a small step, but a step nonetheless.
For most of these games the Griz had Conley hiding out on the other teams' weakest offensive threats (for example, Mo Harkless or Marcus Smart). He rarely went toe-to-toe with the top point guards, at least during the bulk of the game. I certainly wouldn't call him a complete liability on defense, but he could be targeted and made to look very small when switched onto size (physically Rubio had much more of a chance). He has quick hands, good instincts, is smart, and can stay in front (at least in key fourth-quarter situations when they let him take on more talented players), but I worry a bit that he's going to be put in situations with the Jazz that he's not been put in for a while with the Griz. We may not just automatically want him to take the weakest offensive threat and hang out defensively for most of the game.
I certainly don't see the idea that we've been able to upgrade the defense, or that now we have a lock-down defender at PG, as some optimistic pronouncements have said. I see the Rubio-Conley swap as exchanging a top-third or top-quarter defensive point guard for a top-half one.
Offensively, I think Conley is a big upgrade. I love his floaters and that he's a real three point threat. I love his smarts and that he doesn't turn the ball over much. His best passing skill is the little pocket bounce pass off the pick-and-roll. Unfortunately, that's not a great match for Rudy, so I share the concern that (like Rubio) he'll have a real transition in making the vertical pass to Rudy. And I see how the fit with Favors could be great, as Derrick is one of the league's best as dealing with these pocket bounce passes.
So to get back to the original point: I think the impact of exchanging offense for defense (or vice versa) are greatest when the offense-defense balance is out of whack. So exchanging an offensive threat for Favors is likely to have the greatest effect when we're much better at defense than offense. It is less likely to have the same effect when our offense and defensive balance is closer (which I think we've already produced with the Conley move). Maybe we're still defense-heavy compared to our offense, and so the marginal returns for an offensive player might still be worth it, but I don't think the issue is as clear cut as it would have been before acquiring Conley.