It would be impossible to build today by any means. Not because of any technological problem. Physically it could be done rather quickly, as others have posited. The problem is that you could never combine the political and economic will to complete it.

It’s the same reason it would be easier to colonize Mars than to fix climate change. There’s a half dozen robots on Mars that will do whatever you tell them. On Earth, there’s 7 billion assholes who think they have a better idea of what to do.