Funny, the Amex on my Pixel Watch stopped working only a couple weeks after the physical card expiry.

It was quite confusing, because a) I received a replacement physical card several months before the card expiry, so by the time my watch stopped working I'd entirely forgotten about it, b) there's no indication anywhere in the Android/Wear OS of what the expiry date is or that it might be expired and c) there's no indication at the point of sale that the virtual card is expired, simply a generic "Declined" message.

You can run a charge with only the card number if you have sufficient trust. Each additional piece you add reduces liability and transaction fees (add exp, add cvc, add 3ds, ...)

Account Updater functionality isn't necessarily even involved there. In the end whether to accept a transaction is up to the issuer, and quite often they'll keep accepting recurring transactions on otherwise outdated card information.