You are still assuming that infinite systems count as simple versions of real world finite systems

That's not just an assumption; that's the null hypothesis, the default position. Sure, you can challenge it if you want, but if you do, you're going to have to provide some evidence why you think there's going to be a qualitative difference. And even if there is some such difference, it's still unlikely that we're going to get literally zero insights about the problem from studying AIXI. That's an extremely strong absolute claim, and absolute claims are almost always false. Ultimately, if you're going to criticize MIRI's approach, you need to provide some sort of plausible alternative, and right now, unfortunately, it doesn't seem like there are any. As far as I can tell, AIXI is the best way to bet.