Some people think we're on track to approximate the brain with a computer on the order of a couple decades from now. But I wonder if we'll discover it's physically impossible to do that with transistor-based chips at all (density of information, energy limits, etc).

Of course that wouldn't mean AGI itself is impossible - biology's own mechanisms could be recreated artificially - but AGI estimates would be way off if we find out in a few years that actually we have to start over from scratch.