Some people need to be realistic, the docks wouldn't have been filled in if they were busy. Instead of a nice park in the city centre we would have had a barely used marina full of scaggy water and it would be even harder to get from East to West. Same applies to Princes Dock - just because Princes Quay has turned out to be a failure you can't say at the time it was a bad idea. As in football, hindsight is a wonderful thing.
Princes Dock should have been the site of the marina, that's not hindsight, it's common sense and what most people thought at the time the marina was built.