Face it, the real crux of the issue is overhead storage space.
Nobody would care about who was seated first if everyone played by the rules of what a "carry-on" was. When you're pre-assigned your seat, it's no real privilege to board first and sit there for 20 minutes while the rest of the passengers get situated. The privilege is in getting first dibs on the limited overhead space.
I don't know about the other AMEX holders, but *I* always take advantage of their other great perk, which is one free checked bag. 95% of what I brought is tucked away in cargo, where it belongs. What I do bring on to the plane will neatly fit UNDER my seat, even the seat with the metal bar running through it. So boarding early offers no real advantage to me... except to be entertained by the folks jockeying for overhead space.
BTW, if you're gold/platinum SPG and hold the SPG Amex, you can now sign up for the Delta/SPG Crossover Rewards program and board in Zone 2 plus get one free checked bag, along with the other Delta SkyMiles AMEX holders.
https://www.delta.com/profile/spgLanding.action