It just doesn’t seem to make any business sense to turn an app into an API invisible to the user. A visible, established brand is a "moat" as they say. Without user-facing brandable software you’re basically nothing. Only techies are interested or even understand the
concept of a service that only exists as an API.
I also think you’re way overestimating (based on one datapoint?) the number of people who want to use their devices from within an LLM chat app, even if at some point they should become aware of the idea.
If there is a demand for it, people will invent businesses to fill that demand even if a pre-AI company like Spotify is resistant to the change.