There are still physical limits to computation, even if we had godlike powers to rearrange the universe as we like (Bremermann's limit, Landauer's principle, there are probably others but I'm not a computer scientist or physicist). More practically, the mass and energy we have to build and operate computers with is finite. Until we know what principles the brain actually runs on, we can't do the math to determine if its physically possible to build computers that simulate it.

That said, if we find it uses some new principle that we don't exploit in our computers, things get very exciting because then we can start trying to do that (you see a faint whisper of this in the excitement around quantum computing).

I mentioned this elsewhere, but encryption is a great example: other than the breakthrough of another sort (like quantum computing), we can easily come up with a key size that is exponentially harder to compute solutions for compared to computing power increases we can achieve. In a 100, 1000 or a million years.

What if the problems we need to tackle are of a similar complexity? Do we ever get there?

We are all holding our breath for both fusion and quantum computing, and while we know they are theoretically possible, will we ever make them practical?