Still going multiplatform will in most cases create much more total demand, but I wouldn't be surpised if the total demand in that situation is more like 0,75x+0,75y instead of just x+y.
Personally, i think that exclusives (let's use GOW3 for this example, i have it in my hands) have more 'glamour' because Sony's marketing campaign is geared around that. It's an exclusive, it's only on PS3, it's one of the most anticipated games on PS3, it's a fantastic game. Glamour.
Millions of PS3 games will buy the game. Why will they buy a game? Only marketing research will tell us that, but we can make some assumptions. Out of all the 'decision breakers', i just don't see how 'being exclusive' would make someone buy the game. We buy the game because it's a good game, because it has fantastic graphics, some will buy it for the story, some because they don't know what to do with their money. Some will just be at the shop, see the cool cover and go "oooh scary, blood, what's this?? let's try!". Some will only buy it to see how the new funky AA works. LOADS will buy it as a present for someone else.
Now, why will people who own a PS3
not buy the game?
Too much gore. Too similar to old GOW games. Don't like action games. Don't care for the story. It's the third in the series and they don't want to go through two games before this one. Kratos looks like he's on steroids and they don't like that. Kratos's head is too small relative to the body (you know who you are
). The camera angles, this and that, there are hundreds of reasons, and i would think that 5 reasons will take up the vast majority of the total non-sales of the game.
Now let's say the game goes multi-platform (in a parallel dimension where Sony makes games for MS). I just do not believe that 'going multiplatform' is a good reason, compared to all the reasons i could think of above, and all the reasons i couldn't think of, for people not-buying the game. Definitely not enough to reduce the demand by 25%, like in your example (though i know it was just an example, a number thrown out there).
No one on this forum has researched this, and to be honest i have no idea how anyone could possibly tell us whether, historically, demand for a title was reduced when it turned out to be multiplatform.
Out of all the people who wanted to buy Final Fantasy 13, did anyone not buy it because it went multi? No, people did not buy it because they think it's rubbish based on reviews. Or for a host of other different reasons.
Same for 'smaller' games. People don't let those games pass because they're multiplatform, they do so because most of those 'smaller' games are shovelware, not interesting enough, or because they're just not worth people's time.
EDIT: Not-buying a game because it went multi is basically boicotting it. There might be the odd uber-fanboy who does that, but the percentage will be so small it's negligible to even account for it.
Again, that's just me.