Again I think Toyota are the perfect counter-example. They had the double decker diffuser before Brawn did. And lots of money. But it did them little good. Williams had a double diffuser from the start of the season too; it was clearly no guarantee of success.
I think the underlying reason Brawn fell back was lack of money to continue developing the car. If they had decent funding, they could have been developing other aspects of the car while others were scrambling to catch up with the diffuser idea.