Well I agree that we should only omit parens where it's 100% unambiguous where the actual parens would be. There'd need to be a very small number of simple rules/exceptions (such as "every new level of indentation denotes an extra parenthesis unless ____").
A few years ago Paul Graham seemed to think this would work. However, it may be that he tried it and found that it would be too problematic. I'm hoping that he simply has been too busy to implement it so far.