Two games sold 4 million COMBINED across TWO platforms.
The last game sold less than 900k on PS and less than 500k on 360. Assuming a slim number of Xbox owners decide to shift to Playstation, this game will have to massively increase its market appeal to get even close to being the kind of game-changer you're describing. To put this in perspective, the last Halo sold 6 times what the last Dark Souls sold, despite DS being on more platforms. To put this in perspective further, Titanfall has sold more on Xbox 1 than DS has to a market of 160 million consoles.
The last bit is a matter of which threads we each frequent I guess, so I am happy to agree to disagree
And Demon's Souls did 1.69 million. I still don't get why you brought sales up though, I never even mentioned that in the original comment. I don't watch E3 to see which company makes the best decisions for the bottom line or which company has more sales potential. An exclusive souls game will generate much more hype in the core community than anything MS could drop (outside of a huge moneyhate). That's my argument. I'm not saying Souls will sell more than Halo, it's not even comparable.
Souls has a very dedicated fanbase, and it'll no doubt move systems though obviously not on the level of Halo or GT. Most importantly, it'll keep the core's attention on Sony, and I'm of the opinion that the core often have a big impact on how well a console does with the mainstream (more of the core migrates to PS4, they get their friends to go PS4, and so on). Sony has done an excellent job of capturing the core and this only solidifies that.
I guess maybe if MS has Shenmue, that'd be a similarily big hit with the core.