I think system seller is term that needs to be thrown in the bushes. What does it even entail there is no criteria to establish what is or isn't a system seller. Plus the games seen as OMG system seller like Mario 64 and Halo were on consoles that got crushed by systems that supposedly had no system sellers.
Ps4 sales will be up although I don't think by much. Hear lots of talk about Watchdogs and wanting to get a Ps4 to play it. Plus there is MLB that came out. I think 3:1 is a pretty safe bet.
The term System Seller is fine I think the issue is how randomly its applied and defined. Clearly you buy a console to play games, clearly any moderately successful title is to an extent a system seller the question is what factor did it play and at what point does a title become a
notable System Seller (which is what I think most people are using the term for)?
The issue (or some of 'em) is that a lot of people use the term in a very all encompassing way which ignores the many permutations possible - at its simplest it seems to me people use the term thinking the title will ensure success, will sell lots, etc.
Yet as you note Halo and Halo 2 were arguably very notable system sellers for Xbox, yet on their own they couldn't deliver anything like the sales PS2 saw vs Xbox. Wii Sports clearly helped Wii a lot but really I'd say it was the Wii Mote and the experience that sold the Wii big time, Wii Sports was just the most associated title and was packed in a lot. Then there are system sellers that are also multi-platform, essentially helping multiple systems.
Myself I think the big mistake is a lot of people talk about system sellers as the key to success, and exclusives as the key to success. I think the evidence from PS2 generation onwards is that this isn't the case. They help, and can at least help a system get to a respectable level of sales, but I don't think any system seller made PS2 what it was or Wii, it was the timing, the composite whole of the offering and the market perception of those devices. I think we're seeing the same thing right now this gen in terms of how the consoles are performing respective to their library size and supposed "big sellers". Market perception of which console to go with and which is "best" is clearly outweighing individual games and I don't see that changing.
Anyway what was my point? I think basically system seller is okay as a term and valid when thinking about contributing factors for a console but often miss-understood and miss-applied as a term making it nebulous and of lowered worth in many discussion (which is I guess your point to an extent).