So we can go to our customers & say, you’re overspending on your TFL travel, click here to save £250 - one click.
A saving like that would have to be on an annual season ticket - how many of those customers are unable to spend that much money in one go? It might be cheaper across the year, but it’s a heck of a lot of money upfront, especially to people living on narrow margins.
tl;dr Samuel Vimes’ ‘Boots’ theory of socioeconomic unfairness applies.
Yes that’s the tricky part, maybe a Monzo overdraft / loan (though we don’t know if they’ll offer the latter), could be the solution 
I suppose once Monzo has fully launched as a bank, they may be able to offer a loan for season tickets - “Based on your spending, you could save with a season ticket. If you continue to travel on TfL at the rate you are, a Monzo-loan backed season ticket would cost £2 a day instead of £2.50. You just keep touching in/out and they loan will be taken out of your account as you go”
Or more likely, given the marketplace model, basically be an affiliate for TFL and make commission on each season ticket they sell that way. Could even do it at 0% (unlikely though) that way they make money from the commission which may be higher than the % people want to pay on a loan.
Commuter Club would be the obvious choice here rather than TFL themselves, TFL wouldn’t actively block this behaviour as it is in their manifesto to attempt to deliver the best value to consumers. However they make £300m aprox yearly from overspend, so they might drag their heels.
Commuter club on the other hand is a solid private sector alternative with the right incentives, and mondo could agree to a commission structure to boot. Win win.
Naji’s just shared that fact that Monzo are considering offering loans, which would be perfect for this 