How about selling loyalty to businesses?

Your response to these critiques has been awesome too!

The lending will allow Monzo to break even, it’s basically a stop-gap to stop them burning through investors capital, while they get the API & the marketplace ready, which is when they’ll make the real money.

It is a very different approach to the way that banks make money now. Monzo won’t be relying on punitive fees (like unplanned overdraft fees) & selling you products which may or may not be the best products for you, while your inertia prevents you from switching.
Instead, they’ll become the hub for you to use to manage your finances, giving your the tools that you need to analyse & manage your finances with as little friction as possible. Eventually enabling you to choose almost any product, from almost any provider for each of your needs in a competitive marketplace. How does that sound?