These challenger banks are also currently operating in relative niches. Yes Monzo has a million bank accounts, but in terms of their product offering, it isn't

These challenger banks are also currently operating in relative niches. Yes Monzo has a million bank accounts, but in terms of their product offering, it isn't close to the breadth of your traditional "High Street" bank. As their product offering expands, their compliance requirements will become a lot more complex, and will lead to more compliance personnel being hired. They have also yet to experience a massive overhaul to the compliance requirements of their existing systems. I'm sure they are better placed to handle it than a traditional bank but it won't be easy.