I think a lot of people who read so much into the length of a game are still teenagers or don't have proper lives yet. I don't mean that in a derogatory way - when I was 14 I LOVED the Ps1 RPG's like FF etc - because as a kid you didn't have that much money and you could play them for hours upon hours. They were huge value and after school I pretty much had 6+ hours to play games for - when Socom / online came out on PS2 that was even greater.
But now I work 8 hours+ a day, spend half my evenings seeing people so gaming time is at a premium. I hate that fact as I still love gaming but thats what happens when you grow up - and I'm not gonna swap seeing people for gaming. So yanno, when you're in my position, 10 hour games are perfectly 'okay' as well it'd probably take me months to get thorugh a Ni No Kuni. Being able to complete something like Dmc in a fortnight and then move on is nice. Now when you think that the vast majority of gamers are in my situation and that's why no ones striving to make longer games.
To sum it up - 40% of people completed Assassin Creed II's campaign. That's somewhere in the 12 hour ballpark with no online - it sold millions upon millions - and yet the game was too long for the majority of it's players. Once you grow up and have less gaming time - this move to shorter more refined games becomes appealing, stuff like Uncharted nails it where I never really feel like I'm grinding or fighting hoards of enemies to pad out the gameplay time. No it feels like the whole thing has been paced and designed from start to finish to make an experience perfect for that 10 hour window - to me thats far better value then a bazillion random encounters.
And just as an aside - I still love the JRPG and next time I have a week off work will dedicate it to playing through Ni No Kuni like I did as a kid, sat in front of my TV for 6 hours a day. But for the other 48 weeks of my year games like that just arn't viable...