I don't think there is any plan about calling anything a "FF-killer". That's extremely childish, and Final Fantasy is very important to the PS4, especially in Japan. I also don't believe SCE has ever marketed something as a franchise killer (maybe Crash during the days of PS1 or something). Japan Studio has a lot of focus on it at the moment, and a lot of effort is involved in getting that studio to the level of productivity, and also success, that many other SCE studios are currently enjoying. You naturally focus on a potential strength, and as the name of the game is currently digital, and mostly big projects, it's fitting to try something which there is certainly a potential market for. At least, in the eyes of some of those who are in charge.
I see a lot of threads about RPGs from Japan posted on GAF, but pretty much all of them are targeted at certain demographics. They aim small. Part of this is because those developers don't have the money to take risks, but also because they're sticking with what they know works for them. It's an investment, and they make it work for them the way they see fit. The Tales series is an example of this. I certainly don't expect something like that to find huge success worldwide, because it feels limited, and huge success today means selling a lot more than what that series is capable of.
There isn't a need to do that with whatever is being attempted by SCEJ/A. It's going to be a game targeted at a worldwide audience, and Final Fantasy is just the best example of something that keeps a different aesthetic without delving too much in territory which is awkward or off-putting for large amounts of people. It helps that it has a built in fanbase, but when I look at something like Final Fantasy VII, or X, or XII, or even XV to an extent, there's nothing particularly off-putting about these games from a marketing standpoint. That's largely where the comparison comes from (and from a production values standpoint). It's ultimately going to be its own thing. And today, where Final Fantasy isn't releasing every year, there's certainly a greater chance of standing out and potentially earning that success.
Every studio at Sony is experimenting or creating IPs with ideas of future success. Some studios have already achieved this and found their route of success (Polyphony, Media Molecule, Santa Monica, Naughty Dog), and continue to move forward with other projects, whilst others have been moderately successful but will get opportunities to try again with different projects (Guerilla, Sucker Punch, London etc). And of course, there have been those that failed unfortunately. Japan Studio is no different. There was a known problem, which is why they had a change of management. They also need to find their foundation, build some projects that can be lasting, and also have an impact. Once The Last Guardian is out the door, what next? It can't just end with that. This is the moment to attempt things and aim for something different, which can also be successful. There is a strong workforce there, and they can certainly build big projects, they just needed focus and guidance. Not everything has to be small like Knack.
SCEJ/A in general has its head on straight at the moment, and it also helps that they're currently doing a great job of approaching various developers to get other projects off the ground to help support the PS4 in that market whilst their own projects remain farther away, and also provide more variety. Dragon Quest coming back to Playstation seemed laughable last year, but it's happened. And that's just the beginning, even for that series in particular on Playstation. There is a strong focus at the moment from that branch, and that focus, separately, is also something Japan Studio needed in particular. It doesn't mean they'll find success necessarily, or that four or five years from now, there won't be regrets. But addressing and fixing the issues is a start, and there have already been fruits of that labour.