In London I find it's mostly only budget restaurants that don't accept Amex, and I tend to use cash at such places anyway: I've been the victim of card fraud three times in the past four years, and I tend to avoid using any card for small transactions (£30ish or less) as a result.

In other countries things are highly variable. For example, in Brazil it isn't uncommon to find restaurants with exclusivity agreements which only take Amex or only take Mastercard. (Cheques are still much more common in Brazil than here, so it wouldn't be odd for the locals to use those.) I find it is pretty useful to carry around an Amex, a Visa and a Mastercard, as well as having a couple of cards back in the hotel safe.

Is this a general comment or is the OP a friend? It's just that it seems strange to generalize. Some people don't spend more than £20k a year on retail products so couldn't spend that much even if they could put every little grocery bill on Amex. (Indeed, quite a few people have a post-tax income less than this.) And a small but not insignificant number get through more than £20k every month.