I think you're ignoring the 40+ million PS2 owners out there. To many them, the Playstation brand is where they play there games. It was the same with GC and Xbox owners, but obviously, their userbases were far smaller, so the power is not near as large.
Also, I said price is an important factor. Don't think that they're mutually exclusive though. The $400 PS3 vs. the $300/$400 360 outsold it with help of it's brandpower. Check consumer electronics polls, and ask uninformed gamers who owned a PS2, and they think the PS3 is the greatest console out there. I'm not saying it's all gamers, but there's a segment of the population that doesn't follow the console popularity contest, so sales up until now have not had much of an issue. These are also the users that buy mainly multiplatform content: sports games, licensed titles, etc...
But now, the thing is, this brand loyalty will lose some power over time. Why? Well people are getting a 360 for the multiplatform in favor of the better price. Then, they're telling their friends who haven't bought a console yet to pick up a 360 so they can play with them. Since the 360 userbase has expanded faster than the PS3, this word-of-mouth will have a larger effect. Many of these people persuaded by word of mouth might have been PS2 owners, and have been persuaded thanks to the size of the userbase, and the low price point.
The only brand out there with any remaining power to the uninformed is the Playstation brand. Why? Because that's where the bulk of the uninformed gamers were. The original Xbox had mostly gamers that were interested in Xbox specific franchises. Sure, there were people who bought an Xbox for sports games, but most bought it for Halo. The bulk of these people have already moved on to the 360. The Sony systems have far less weight on their exclusives. Yes, their exclusives sell decently well, but not to the extent that Halo did on the 360. Because of that, these users are simply tied to the Playstation brand, and not the games themselves. This means that, while these users would prefer to keep with the Playstation brand, they're not attached to any Playstation franchises that will tie them to that system exclusively. This makes it dangerous for Sony, because these are users that can easily be persuaded to move on to the 360. And with the current userbase and price point for the 360, they're much more likely to do so.
P.S. I haven't really thought this post through, and was mostly brainstorming as I was writing, so there may be some conflicting statements...I don't know. :lol