You handled the situation exactly right. There is no perfect response some people want more updates some people want less - what you go with is what feels right for the product and brand you are representing and striving to be.
If anybody values the plastic method of payment should always carry 2 cards just like if you are in a car you should always carry a spare tyre Anybody complaining of such inconvience should look at their own preparation.
Assuming 100% have smart phones then we always have Apple / Android Pay to act as a safety net as I did today.
Probably a good learning for what really is a new team at Monzo and an emergency is good for all people to go through at least once.
As always keep up the excellent work