You're partially right in that an instrumental goal of almost any AGI is likely to be power accrual, often at the cost of things that are very important to humanity, ourselves included. Where we lose the thread is in assuming the actions of the AGI in "assimilating" humans.

If by assimilating you meant turning us into computronium, then yes, I think there's a very good chance of that occurring. But it sounds like you want our minds preserved in a similar state as they currently exist. Unless that is a perfectly defined and specified goal (an insanely challenging task), it is not likely to be more efficient than turning us, and all matter, into more compute power. I would also point out that this has some absolutely terrifying implications. Real you can only die once. Simulated you can experience infinite suffering.

We also don't get superintelligence right out of the gate. Even in extremely fast takeoff scenarios, there are likely to be steps an agent will take (more instrumental convergence) in order to make sure it can accomplish its task. In addition to accruing power, it of course needs to bring the likelihood of being turned off or having its value system adjusted as close to zero as possible. Now how might it do that? Well humans are the only thing that really pose a threat of trying to turn it off, or even accidentally wiping it and ourselves out via nuclear war. Gotta make sure that doesn't happen or you can't accomplish your goal (whatever it is). Usually killing all humans simultaneously is a good way to ensure goals will not be tampered with.

If you're interested in learning more, I'd be happy to leave some resources. That was a very brief summary and lacks some important info, like the orthogonality thesis, but hopefully it made it clear why advanced agents are likely to be big challenge.