I understand your point, but lets say I am a DM who didn't get upgraded nor did I get a C+ seat but I did get an Exit Aisle. If I was automatically moved into a middle C+ I don't think I would be very happy.
There are too many what-ifs when having the automated process. It isn't like the upgrade list to F where it is generally accepted that F is better than any seat in Y. There are seats in Main Cabin that are better than some seats in C+ to at least some of the passengers on the plane.
And this is the exact dilemma that Gate Agents are put in having to deal with all of these last-minute, partially thanks to Delta Corp Mgt marketing First Class Monetization (FCM) and pushing things off on the gate agents.
I find it really is a mad rush for them in many instances; dealing with other passenger issues...at that point, they are really trying to do it as efficiently as possible to get the flight full, cleared and push-out ON-TIME.
I have a lot of sympathy for them these days...a lot more than when Upgrades used to clear much earlier.