I think GP's point was that with AMEX not being accepted everywhere, it would no longer be a transparent action to pay with tap. Now they would have to either deal with the rejection and change to a non-default card, or they would need to prepare in advance by looking around for what kinds of cards are accepted. Neither of these things are a big deal, but it's friction, and consumers don't like friction.
Yeah I mean if it happens once or twice I’m gonna switch to a different default card. The .5% extra cash back isn’t that much, especially for something that’s all about convenience.
Mostly the Apple Card was a novelty for me: 100% of my usages is via Apple Pay.