Sony haven't "lost" anything, (besides Ace Combat 6) as the PlayStation still got those other games, they just had to share them. And realistically, the exclusives that went multi haven't really been the PS3's greatest tragedy, it was the price, but that's another time.
For Final Fantasy VII though, I could see a fight for it, mainly due to the conditions I mentioned earlier. But yes, of course Square-Enix could put it on whatever they want.
If we're talking about the PS3 specifically, DQ isn't on it, MH isn't on it, Fatal Frame isn't on it. And once exclusive franchises are now multiplatform like FF XIII and DMC4 and whatever. Can't say I think FF VII would be an exception when I'd say FF XIII was more important to them.
- Fatal Frame wasn't an exclusive to begin with, it was on the Xbox as well, though the sequels were not.
- Monster Hunter would have done well on the PS3, but the PS3 isn't hurt without it.
- Dragon Quest is the biggie. DQX would have done very well on PS3 in Japan. In the West, not so much.
But those may have have stayed with the PayStation had it not been for the high price point. I know you were talking a lot of exclusives in general, but the ones that hurt were Final Fantasy, GTA, & Devil May Cry.
Looking at FFXIII's generational decline (~2.25m to ~1.85m), MH3 and DQIX both likely would've done worse on PS3 than where they went though. And they likely would've done worse in the west too, since I doubt we see the sort of push from Sony that Nintendo's giving them.
Not to mention SquareEnix wouldn't have done jack to help Dragon Quest itself. It would have bombed in the west just as hard if it was on PS3 as it has on the DS. So pretty much, Nintendo is the only one that is helping Dragon Quest outside of Japan at this moment. While SquareEnix is just focusing on Final Fantasy (and its action RPG cousin Kingdom Hearts).
In Japan however, I daresay the sales for Dragon Quest X would have been lower due to the smaller install base. There's a reason Yuji Horii always says he puts Dragon Quest on the 'most popular system'. Last gen it was PS2. This gen its DS and Wii.