If the Amex partnership includes global (or at least, European) expansion (which seems more plausible than with GS), it may generate extra pressure on vendors to accept it. iPhone market share is quite high in Europe(~35%), and I’d guess mostly comes from high spending sectors (e.g. 56% in Swirzerland, 50% in UK). I would try to attract their business.

It would definitely help Amex, but I'm not convinced that Apple would inflict the significantly worse acceptance on themselves without significant upside.

That upside might be Amex's uncapped EU credit card interchange, but that would very likely not apply to a co-branding scheme with Apple: https://www.headforpoints.com/2018/02/08/american-express-eu...

The Apple card was only available in USA. So market share abroad is not really relevant. Card margins are much lower elsewhere so its not such an attractive product.