I never use my Monzo card, or any other bank card for that matter. I use Apple Pay for everything (mostly for the convenience).
My card is a waste of resources, if I had the option I wouldn’t have got it.
During signup I think you should be asked whether you need a card, with the ability to opt out of receiving a card. Of course if you needed your card at a future date you could simply order one (they come so fast).
I simply think that Monzo is churning out a huge amount of plastic to people who (generally speaking) are much more technically savvy and therefore less likely to need a card.
The only feature that I can see being a problem is cash withdrawal, but cash is rarely needed and MasterCard is rolling out Contactless ATMs in the near future: Emergency Cash via ATM network