Ceasing to dream big isn't an improvement. But ya... it's probably for the best the project didn't happen (yet:).

What is "pretty fucked" about this?

Dude, it's totally fucked. Like removing the sea from seaside towns. i'm sure if you're a mediterranean fisherman you would not be happy having the sea forcibly moved away from your home... The environmental impact is probably immeasurable.

How is this any worse than normal dam construction, apart from scale?

How is a flood worse than a drop of water, apart from scale?

Wise words.

Scale.

If the engineering effort fails, an incalculable amount of life and property would be destroyed. Even if it succeeds, a huge amount of costly adjustment would be required by a very, very large number of people.

There would be some pretty vast ecological repercussions. Fish habitats and coastal ecosystems, would be immediately affected. Then, connecting rivers will cut deeper, unforeseeably altering each and every environment they affect.