I think you simplify things way to much...
Granted, I can't account for every single persons reasoning for why they chose this console over that one. Nobody can (with a serious investment in researching door to door worldwide!).
All one can do is summarize based on facts that are known and from there parse out what has the most cause and effect on sales.
Games consoles serve a purpose, to play games (these days expanding out to other entertainment value but I digress).
1) What games are available?
2) How does the game experience compare to other consoles?
3) How much does the experience cost?
4) Can I share this experience with friends?
These are the main factors as far as I can see.
Where is that "brand/image" in that list you say? #4 on the list I think encompasses the majority of the brand/image although some of it does creep into #1 of inferred/expected games based on the image/brand. If your friends would deem it a shame to have console-x, that may deter you from owning it for whatever reason.
This generation has thrown in a new paradigm of questioning reliability that wasn't there before due to the publicity of RRoD, but in all, I don't think that will be an issue next-gen (though it may stall purchases until reliability is proven for a certain populace).
Are there others? Sure, but seriously this is what makes a "cake". from there you can have spice cake, rum cake, fruit cake, all of which are slivers of the majority factor of "cake" and while I'm sure they do add up to amount to
something we can't possibly factor in every iota of data to account for in determining why sales went one way or another.
In response to my list, do you disagree with my summary/reasoning for each of those outcomes? If so I'd love to hear your reasoning!