Yeah...no.
So I started off with looking up the definition of gritty. So I'm guessing the demand is uncompromising realism. I let that sink in for a moment. Uncompromising realism in a JRPG...
No.
Just no.
I have no problem with realism in itself. It's a matter of personal taste, really. If you want a "realistic" game, okay. But no. Persona 4 was not in the strictest sense "real," but then if they did make it "real", they could've lost the true meaning behind what they were trying to do. It's supposed to be a psychological game, in terms of symbolism. Losing the fantasy aspect could very easily have cost it the symbolism that is a very strong part of it core structure.
Final Fantasy has the same thing, but it's not as strong there as it is for the Shin Megami Tensei series because it doesn't tie into the stories throughout the series as much. But it is there.
On the other hand, games such as Dragon Age and Mass Effect aren't gritty, either. Yeah, they're dark, but there are a number of JRPGs that are very dark as well. But these such games share a fantasy setting. Okay...Mass Effect is Sci-Fi, but it's still not "real." You have magic and biotics, respectively. You tell me how that's gritty.
The only thing that could strike a chord as very as gritty goes at this point is the art style, which is anime for JRPGs, but that in itself as largely cultural, as well as a matter of personal taste. In that case, asking JRPGs to be gritty is unreasonable, because you're asking the developers to surrender something cultural for the sake of popularity. If you or anyone else does not like, as others have put it, seeing "emo androgynous teenagers with bad fashion sense", then that's just your taste. It is not your job to slap others with your opinion.
As for why JRPGs are in some decline, I can only imagine that it's because JRPGs are more story-driven and that does not strike a chord with the more action-oriented Western audience. Mind you, that's just my guess, but between that and your vision of the need for "gritty," I'd say mine's more likely.