My estimate is more on the "billions of years" timescale. What aliens one might meet is important, potentially life-threatening information, and humans are a big, important and deep clue about the topic that would be difficult to exhaust.
Being inexhaustible, even if true, is not enough. Keeping humans around (or simulated) would have to be a better use of resources (marginally) than anything else the AGI could think of. That's a strong claim; why do you think that?
Also, if AIs replace humans in the course of history, then arguably studying other AIs would be an even bigger clue to possible aliens. And AIs can be much more diverse than humans, so there would be more to study.
As Luke mentioned, I am in the process of writing "Responses to Catastrophic AGI Risk": A journal-bound summary of the AI risk problem, and a taxonomy of the societal proposals (e.g. denial of the risk, no action, legal and economic controls, differential technological development) and AI design proposals (e.g. AI confinement, chaining, Oracle AI, FAI) that have been made.
One of the categories is "They Will Need Us" - claims that AI is no big risk, because AI will always have a need of something that humans have, and that they will therefore preserve us. Currently this section is pretty empty:
But I'm certain that I've heard this claim made more often than in just those two sources. Does anyone remember having seen such arguments somewhere else? While "academically reputable" sources (papers, books) are preferred, blog posts and websites are fine as well.
Note that this claim is distinct from the claim that (due to general economic theory) it's more beneficial for the AIs to trade with us than to destroy us. We already have enough citations for that argument, what we're looking for are arguments saying that destroying humans would mean losing something essentially irreplaceable.