He has been Brees QB coach for years so if nothing else he should be very good for Dak. The Saints are also a modern, high production offense that he's been involved with.
Maybe he's not the best candidate but I think our offense improves if he comes onboard. Add in all the other stuff the OP mentioned and I think there's a strong chance that Lombardi is in the plans, for better or worse. This move has JJ written all over it.
I think that the Campbell is a good coach. I just don't think he would be able to call plays and draw up a scheme that can utilize Dak's game style. Garrett is not an X and Os type of guy. So is Campbell. In fact, him and Garrett have similar styles.
What we need is a play-caller that knows how to use Dak. Kubiak, Haley or any of the other coaches from the final four teams would suffice. Heck, I'd rather have an innovative college coach.
If Campbell was replacing Garrett than I would be fine with that. But he'll be given the task to call plays and draw up a scheme. I don't want to use that same old tired playbook over again.