DL Plat's and Diamond's basically get C+ at booking. While it's possible there may not be inventory in the C+ upgrade bucket at booking time, it's pretty rare. In the one case I experienced it, there was just one C+ seat left and it was a middle seat (so I wouldn't have taken it anyway).
Yup, my experience as a DL Plat (until the clock hits midnight tonight, when I become Diamond) over the past 18 months has been the same. And as long as I'm logged into when searching Delta I can click the upgrade eligibility button to know for sure whether the instant upgrade is available (and of course view seats to make sure there is a non-middle available). It's slightly more cumbersome than the AA version of just picking any available seat, but I don't find it burdensome. That said, I also don't really see the benefits for AA to go down this road either.