> The paper does not prove anything about the upper limit
Nothing can prove how many qubits can be realizable except trying to realize them. There will never be a theorem that says "The maximum number of qubits that can ever be controlled in a lab is X". That's what experiment is for.
I will say, it's difficult to doubt that the upper limit to the number of qubits we can realize is infinity. We can now trap ~10 million atoms and efficiently control hundreds of them: https://www.nature.com/articles/s41586-023-06927-3. The question is not "Could we ever realize billions of qubits?". It's "When can we realize billions qubits?". The answer could be decades or centuries but as long as people are building these devices, it will happen eventually.
Nothing can prove how many qubits can be realizable except trying to realize them. There will never be a theorem that says "The maximum number of qubits that can ever be controlled in a lab is X". That's what experiment is for.
I will say, it's difficult to doubt that the upper limit to the number of qubits we can realize is infinity. We can now trap ~10 million atoms and efficiently control hundreds of them: https://www.nature.com/articles/s41586-023-06927-3. The question is not "Could we ever realize billions of qubits?". It's "When can we realize billions qubits?". The answer could be decades or centuries but as long as people are building these devices, it will happen eventually.