Sure, but you then can't access the computronium, because communication is so bad.
Also, I wonder if good old-fashioned computing is interesting at all to a civilization that's had access to advanced AI and quantum computing for a while.
Like we haven't really figured out how to get an ML model to run on a quantum computer, or how to build a quantum-native computer (i.e. surface of a black hole, or some other way that doesn't rely on our current sense of quantum error correction), but I don't know of any physical laws that preclude it.
I'd bet if aliens invaded our galaxy, they'd go for the super black holes in the center, or some other resource beyond our use and understanding, not this random water planet on the edge.
> Sure, but you then can't access the computronium, because communication is so bad.
Who says all the computronium has to stay far away? Just move it all closer together as you build it.
Second, you can spend much of it running parallel tasks that don’t need to synchronize or communicate often. You could run quadrillions of humans instead of enlarging yourself to fill all of that new computronium.
You don't access the computronium. You move into it.
"good old-fashioned computing is interesting at all"
"Computronium" is defined as "the best computing power available". I deliberately selected it as a neutral term that does not depend on any particular model of QM or black holes or anything else.
Personally I doubt it's exactly one thing because optimizing for different types of computing is likely to result in a spectrum of computroniums rather than just the one, but the term flexes to encompass that easily enough.
The point is, you build something in that system over there for the same reason a normal human might buy a bit of property and put a house on it. The human in question isn't going "oh, I don't need to do that because the world already has hundreds of millions of residences". The human does that so that the residence belongs to them. The hundreds of millions of residences that do not belong to them do not factor into that question.
