Winning mentality, success breeds success, winning is a habit, or whatever other tired old cliché you want to trot out, there is definitely some truth in it. Being crap in the 6 Nations is not the way to prepare to win a World Cup, it's just Eddie's excuse for piss poor performances year after year. Being below par with some evidence of progress towards a longer term goal I could stomach, but there is zero evidence of progress and Eddie is simply trying to pull the wool over the eyes of the media and supporters by claiming it's all part of his grand plan for the WC.
SCW's England knew how to win a game of rugby, and to do so emphatically. Yes, they fell at the final hurdle in pursuit of a Grand Slam a few times, but these were exceptions, put it down to pressure, expectation or whatever else, but that certainly didn't make them a bad team in anyone's eyes. The point is they were good enough to be in that position consistently for several years, and they overcame those setbacks and were, for a time, unbeatable.