A la carte data can support any position. The most powerful console hasn't always won, but sometimes it has. Being first to market doesn't always assure victory, but sometimes it does.
But it's a mistake to say that, because these facets don't singularly guarantee victory, they have no impact. If power, timing, and first party exclusives didn't matter, you could release the NES today and reasonably expect it to go toe to toe with the PS4 and XBoxOne.
But then, apart from the exclusives, that's pretty much what we've seen Nintendo do with the Switch: old, bland tablet hardware, with nice controllers and TV out, released in the middle of the generation. And it's doing reeeeeally well.
We don't really have any firm rules. Of course we don't, otherwise everyone would follow them, and Atari, Sega, Nintendo, Sony, and Microsoft would have been first-equal for the past 20 years.