I don’t know. If they make a contribution of a few quid a month, it’s their own choice. If they are happy to get a different card as a reward for that, but to know they are supporting the development of features etc, then let them.
I sometimes feel like I should be paying for monzo. The features are insane and I feel bad that I totally benefit from the features without having to spend a penny to access them. Call me too nice… too British 
I guess it comes back to Monzo’s eco system - they have customers on side who become brand champions. I don’t feel like this with any other company… maybe I’m part of the cult