Yes, this is how I see it. You get an increase in awareness that can drive moment for a bit, just like pushing a go-kart, but normally, no title is enough to sustain a console for an entire generation and sell tens of millions of units. A 'killer app' as defined by a game with a lot of appeal is far more important at the beginning of a console's life to whack the install base up quickly. It's also nice to have throughout. But console sales are really dependent on steady growth. For example, what is the killer app on iPhone? It's actually a service of a huge library of variety. Anyone can find something that suits their personal interest. Compare that with a hypothetical Game-and-Watch Gears of War that can play Gears 3 in console quality on the go, but can only play that game and no other. Which will sell the most, the one with huge killer app or the one offering limitless variety?
PS3 didn't have a single title that attracted millions of buyers. Some will say the killer app was BluRay, which attracted the first few million buyers. I personally doubt any game could have been a killer app able to sell 10 million units of a $600 console, especially when the HD console experience was a year old. If PS3 launched with KZ2 a year before XB360 and Gears, I dare say the roles would be reversed, but the economics and technology just couldn't ever manage that. Guerilla and ND and Insomniac, and every other developer, needs time with the machine to make it sing.
Perhaps it is in the console companies' interst to model a new machine around the existing one so development can start now on a killer game that introduces the next generation with a huge fanfare? Even then, a 10 million unit headstart doesn't win you anything on its own. There are too many factors. Targetting a killer game will improve the start but take away from investment in other parts of the development chain.