That's absolutely true, and part of what I'd consider exceptions. It's better to judge franchise sales per platform rather than individual titles, to gauge interest in a game. Unfortunately I don't have that info. I can only cite DMC on PS2, though it's said that interest has dwindled over the years.... This is the time when console are trying to build a critical mass, and this is the time when notable exclusives are the difference maker.
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The problem with the critical mass argument is primarily one of console price. You're not going to sell 40 million consoles at $400, no matter what software you have. The exclusive titles aren't anywhere near as important as the console price. Key titles are essential to convincing pundits your console is worth the money, as GeOW successfully demonstrates. Such strong titles are few and far between though. They need a very strong following who don't care about price. FF may well attract a few million console buyers, willing to spend $hundreds just to play that one title. Most games don't have that though. VT5 hasn't had that. I don't know that DMC has that. That's where the franchise figures come in. Over time, less and less people have been interested in getting DMC, so is it likely to sell many people a $500-600 PS3? If not, it's exclusivity is diluted, even early on where titles have more importance.