Why would it scale with the square? A brain is a physical system, and computers can simulate physics in a way that scales. In principle it might have to be a quantum computer, but most people who've looked into that are very skeptical.
A better argument is that neurons do complicated things. But they also satisfy many inessential requirements in having to be built by evolved machinery and to stay alive; a designed system with more advanced technology than ours could do at least as well at thinking while dropping the inessential needs. So that's certainly possible. This leaves "not easily achievable". It only has to be accessible enough for someone; I-can't-imagine-how isn't a strong argument.
A better argument is that neurons do complicated things. But they also satisfy many inessential requirements in having to be built by evolved machinery and to stay alive; a designed system with more advanced technology than ours could do at least as well at thinking while dropping the inessential needs. So that's certainly possible. This leaves "not easily achievable". It only has to be accessible enough for someone; I-can't-imagine-how isn't a strong argument.