To better explain myself, Monzo needs to achieve economics of scale in order to be profitable. If they make a paid product which has broad appeal to millions of users then the economics of scale help them keep costs down and compete on price as you can be profitable with small margin per paying user if there’s millions of those paying.
But if they make a product which only appeals to a small minority of hardcore Monzo fans then not enough people will sign up for it so they won’t be able to offer good competitive pricing while still making profit.