I’m not saying that I know what are the reasons behind Resolut’s decision. But what about this: First: it’s not just about spending. It is also about exchanging. Revolut might not want to become a currency exchange trader platform. Second: the income generated by merchant fees might not be enough to establish a sustainable business. Revolut needs some sources of income. Every exchange is attached with a small expense Revolt has to cover. Holding and exchanging funds on accounts does not come for free for the service provider.
So, the first rational behind the policy was explained by Revolut staff here: They don’t want to make it too complicated, so they don’t want to make a difference between just exchanging and spending in currencies that need to be exchanged while spent with the card. Therefore one limit and fee to cover it all.
Furthermore, they might want to keep the product attractive and free for most of their customers that might not be big spenders. Instead of asking for monthly / yearly fees like banks or coming up with other hidden – or not so hidden – fees, they came up with something where people that spend or exchange more than 5000 GBP a month end up paying fees that are still pretty competitive.
It is not just that big spenders can afford these fees more easily than a backpacking student, Revolut also charges the people that profit most from their product, because, obviously, if you spend 5000 GBP or more in foreign currencies, you save a lot more money with Revlout than the backpacking student.
It’s impossible to say for me which scenario would generate more income in the long term: merchant fees from people that spend high sums (like you assume) or applying some fees on certain services for people that use the product more extensively. I assume that Revolut has thought this through thoroughly.