Platinum also offers return protection that's not offered on the Gold card (and the Green card has far less coverage overall)... but to your point, that's not something you're going to use for dining.

There's always additional rewards to pick up. A better card for gas, a better card for hotel X, hotel Y, a better card for a specific merchant, etc. Sort of like you can always work more hours to make more money, at a certain point this all starts to become self defeating. Whatever the right balance is between effort and return, I think we would tend to agree it's probably more than 1 card, but definitely less than 13.

I think a single Amex card probably takes things too far; no matter what you'd have to have a VISA/MC just because Amex doesn't have universal acceptance. For me the "good enough" minimum combo would probably end up being the Platinum card + Costco VISA. 3% on dining and travel (flights stay on the plat for 5x), 4% on gas, 2% at Costco, 1% everywhere else Amex isn't accepted with no FTF is probably a good enough lift, and the card doubles as the membership card I'd have to carry anyway. Gets me the purchase protection features for standard purchases, slightly higher earning rates in higher-spend categories for places where I don't need those protections.