They're not even substitutes; RTK Query by default wraps fetch or can be configured to use your preferred http client. So it's an entirely additional layer of cacheing and refetching logic with a standardized interface around common loading and error states.

It is considerably easier to write performant web applications with a library like RTK Query or TanStack Query than to just use fetch and try to roll your own cacheing solution. I say this as someone who has done all of the above professionally.