How many Amex holders don't have a second card? I've never heard of anybody getting to the checkout line, then saying oh drat, they only take visa, and walking out.
For 16 or so years Costco required you to have an AmexEx card if you weren't paying with cash or debit, now they only take Visa. This was apparently something lots of newish members weren't aware of so it wasn't too uncommon to be in line and have someone say they didn't have the right card to pay. You'd then have to wait for the checkout person to give them the Costco Amex card spiel.
I'm sure lots of people use their Amex preferentially, but I've never seen anyone put their hand on a door handle, then see that only visa/mc stickers are in the window, and walk away.
I have. When someone wanted to expense something. Or when they didn’t want to bother with the Chase card declining because it forgot they were still overseas or whatnot.