Re: You don't need to know how 1,600 services work
I think the most valuable thing here is that the microservice architecture enforces encapsulation. Monolithic software doesn't, unless you do so by design. As someone who has to maintain software that began its life in the '80s in a language that doesn't even really have a concept of encapsulation, I can't understate how important encapsulation is for maintainability.
It also sounds like it makes scalability and resilience easier. Good to see also that Monzo recognise the adage that premature optimisation is the root of all evil.