I assume there’s got to be a ground universe somewhere in the chain.
I’m not saying you’re wrong to think this is likely, but I don’t think this is as necessary a condition as some people are taking it to be. So long as each simulation is simulated from somewhere, there’s no reason why it can’t be the case that every simulator is also simulated. I can think of no reason why the universe would be like this, but I can also think of no reason why it can’t be that way.
I’m not saying you’re wrong to think this is likely, but I don’t think this is as necessary a condition as some people are taking it to be. So long as each simulation is simulated from somewhere, there’s no reason why it can’t be the case that every simulator is also simulated. I can think of no reason why the universe would be like this, but I can also think of no reason why it can’t be that way.