Cooling is definitely a problem is your aim is to cool down the millions of physical qubits required to get the thousands of logical qubits needed to do an actually useful quantum computation. No one has scaled up their qubit technology to achieve millions of physical qubits, and this is going to be a very big computer with tons of thermal mass that needs to be cooled down sub-kelvin. That is a substantial engineering challenge.
Qubits are getting smaller at the same time their coherence times are getting longer. The millions of qubits figure assumes pretty crappy coherence times. I don’t see any reason why you wouldn’t fit a future QPU on a 4 inch wafer or some kind of stack of them.
Finally, again, dil fridges are a solved problem. Not only are they solved but there’s a ton of room for improvement. They’re very inefficient. You can just make bigger ones with more dil units.
That leaves the wiring, but you remove a lot of that with cryosilicon computers and multiplexers.
