As an example, Monzo was quite outspoken about their API at first, but it has since taken a backseat, presumably because they didn't see a huge need for it.
You're not wrong.. but it's still very much functional and has received some recent additions (receipt API). I do wish they'd do more with it, though. I suspect the problem is that most customers just don't care.
The internal API is fairly trivial to begin using, if necessary.
IMO the reason is they (perfectly rationally) realised after spending several million building an authorised bank that APIs are a pure downside scenario for them, i.e. intermediation risk. Why spend so much money to stand up a de novo bank and let everyone else capture the value? 99% of the value is top of stack. This is why no market in the world has provided 1st party banking APIs for retail products. It's taken regulators to do so in Europe and the results of that (predictably) fell short of what everybody had hoped. No criticism of Monzo for this. Based on their incentives this was the rational course of action.
I suspect there are is a decent number of 'techies' and early adopters who found Monzo's API, despite its limitations, attractive to them. I've used it to save myself a bunch of time with expenses and it's nice to know that, if I wanted to, I could use the internal API too.
In its current state, Monzo could easily expand the existing personal API (which deliberately doesn't allow you to scale - only whitelisted user IDs can use your OAuth client) whilst minimising intermediation risk, provided that they didn't add the same endpoints to AISP API.
Maybe it's niche, but the number of programmers in the world continues to grow and if there was one that would be a powerful differentiator among that audience.
Currently banks have been implementing API’s because it has been mandated by regulatory bodies. From their perspectives, the APIs are a money sink, and even worse, could make their customers less reliant on their infrastructure - increasing their chances of losing customers.
I see it differently, and predict that the landscape will change dramatically. Open banking APIs are going to enable a whole host of products and services - allowing customers to verify their income painlessly, or manage their investments and credit cards with minimal effort.
As the ecosystem of products around open banking grows, banks will begin to see the quality of their APIs as a strength, and a way to differentiate themselves from competitors. Customers could one day choose their bank based on how well it integrates with their smart phone, or favourite online market place, etc
Monzo are already capitalising on the opportunity with some very smart moves. They are using https://truelayer.com (a banking infrastructure company) to display their customers credit card’s balance and transactions within the Monzo app. Eventually users will be able to pay off their credit cards from within the Monzo app. Just imagine the possibilities!
In short, there are many exciting opportunities, and early adopters could win big.
Build products that don't break every 5 minutes because they don't depend on sticky tape and bubblegum holding them together (screen scraping)
For example, use all unspent money at the end of each month to buy shares.