You can purchase at least some types or Airport Express train/Octopus public transportation cards with a credit card, including AmEx, at the airport, but you might have some unused credit on it at the end of the day.

You need cash for street food and similar purchases. I don't remember whether museums take credit cards, but major and hotel restaurants (more expensive) with English menus and English spoken should take all credit cards. I think taxis may take the octopus card, but I can't remember using it that way. Similarly, I've always paid for the star ferry (do not miss) with coins from my hotel, but they must accept the octopus card for the fare.

I would be uncomfortable walking around for a day without any local cash, but that's me and I can always save it for the next trip.