It seems like a lot of people are taking this all to mean "I want all JRPGs to be short," as opposed to simply wanting more variety in length. I love JRPGs, but I think a lot if people have a really narrow view of what they could be. I think most people asking about this are asking for the novella equivalent of a JRPG - short and sweet but with a much more focused scope. Just as shorter stories don't immediately invalidate more longer ones, shorter JRPGs would only serve to provide more variety in the types of experiences available.
I agree that as it stands, the typical JRPG formula generally doesn't lend itself towards more consolidated experiences, but there would be nothing stopping the game in question from bucking tradition a bit to accommodate the smaller scope without feeling neutered. Barkley Shut Up and Jam is one of my favorite JRPGs, and I can easily beat it in five hours. Not every story needs to be a lofty tale of saving the world. Ten hours is too long to develop a cast of characters? There are plenty of ways to make a story with only one character, even if they are more psychological in nature, but even so, how many purely psychological RPGs are there?
Constraint breeds creativity, and not the other way around. A good game designer would find a way to make the JRPG formula fit the timeframe, even if the entirety of the usual convention wasn't intact. It's very well possible that these sorts of experiments would create new mechanics, new conventions, that could be adopted by their more longer counterparts, wholesale or with some modifications.
At the end of the day, I think it's because the tradition is hard to escape completely. People have such a pretty well-defined of what being a JRPG entails that they're not willing to do more than provide a mere twist on what's already been established. I'm kinda bummed out that the rise of indie games hasn't really done much in the way of being more experimental, taking the idea of a game jam and extrapolating it to a more longer experience.
Hell, on the other hand, why don't we have a JRPG equivalent to the Odyssey, taking 200+ hours minimum to finish its main quest alone? Variety is the spice of life, and I can't see how extending that to JRPGs can be anything but a good thing, assuming it doesn't come at the expense of the format people clearly already know and love.