In my experience this is only true iff the system never gets any more user facing features. Every new non-insignificant feature requires a new REST-API route,

In my experience this is only true iff the system never gets any more user facing features. Every new non-insignificant feature requires a new REST-API route, or database table or modification of the GraphQL schema. And depending on how you designed the backend, even small redesigns of the frontend might require changes on the backend. Consider a simple app showing car rentals, where you initially have something like /car/[id], then the frontend guys realise that we need to show cars rented by each customer and it's necessary to have /customer/[id]/rentals. (Of course it's possible to design a system without any schemas or normalisations, which would make your statement truer, but that's rarely attractive for other reasons.)