Hitchcock is a great example, and I'm glad you brought him up, of someone who managed to find an ingenious way of maintaining final control. He would storyboard the shit out of everything, and seldom deviate from it. Essentially, like Spielberg, the film would already be edited in his mind before the first foof of film went through the camera.
This prevented the studio from jumping in and altering his "vision" because there was no footage shot that he didn't want. Pretty smart guy. Spielberg did exactly the same thing, more or less, and it wasn't until Schindler's List that he deviated from the storyboard.
Now, this is a bit of a double edged sword, especially in the case of Lucas. Professional editors are trained to tell a story in 120 minutes or so, in the most effective and efficient way. They know what works - and then you get some director coming in and messing around with it. It flops or doesn't work, and the editing gets blamed. It must have sucked to be Ben Burtt from 1999-2004.
These days, anyone can shoot a film. Mini-DV cameras are so damn cheap now, and most PC's or MACS can run high end editing software. Armed with a good script and some decent actors and someone who knows a little craft, there's nothing stopping someone from producing a really great flick this way. It's basically the way Rodridguez (sp?), Linklater and even Tarantino started in the 90's.
No one is ever going to produce Armageddon 2 - The Return Of The Rock on a $500 budget, but damnit anything is possible with the right amount of skill and ambition.
EDIT - I realize I spelled Lucas wrong - then I looked at it and kept it 'cause it was funny.