currently I cannot find a reason to use Duban over Ochain as the stats currently are. Though this info may change drastically here soon as the next stage is due in a few weeks.
I pretty much agree with your whole post, but to be clear, the biggest reason right now to use Duban over Ochain is for anyone who doesn't already have a completed Ochain. There's really no good reason now to make an Ochain if you didn't already have it, other that completionism, given the ease of getting a Duban. Pretty damn nice for people like me, who came to PLD more recently and have yet to make an Empy.
Duban is a perfectly fine Ochain alternative in any situation, with the advantage of better block rate on higher level mobs, and the disadvantage of not having MP return (which I would argue is normally pretty minor, given the many sources of refresh from party members that you are likely to have available, damage>MP gear, and Chivalry).
And yeah, with any significant improvement on next stage, Prime Shield seems very likely to totally replace Ochain (and maybe kill Srivatsa too). Prime+Aegis seems like all you'll really need in the shield slot for any defensive situation, toss a Blurred+1 in there for deeps.