Unless you end up somewhere that doesn't support ApplePay, and doesn't accept AmEx. It's uncommon (where I live, at least), but it definitely happens.

That's why the one card I carry is Visa. If apply pay doesn't work, then I don't get the biggest % reward, but it doesn't matter that much to me.

I assume that's why the parent said there's little need to carry around more than one

But if your 'one' is the '6% on groceries' AmEx, you do need to carry more than one (or accept that there will be places you can't spend money).

That card is still accepted anywhere Amex is accepted. But yes, if you want to cover vendors who don't accept Apple/Google Pay or Amex, then you would need to carry multiple cards.