Yes, it's so successful that SCE has never bought a single Japanese development partner despite success stories.
The truth is SCE only has SCE Japan Studio, and the two Polyphony branches in Japan. They help start companies like Level-5 and Shift, but apparently lose interest and never buy them.
Prior to the PS3-era, from Sony's perspective, there was never any need to buy them though.
They were basically making mostly if not entirely Playstation-exclusives. Aside from the occasional publishing/funding, it seemed like smooth sailing for the nature of the existing co-development stance that SCEJ and the other 3rd-parties had.
Sony did not foresee the huge hurdles and challenges all their 3rd-party partners were going through during the transition to HD gaming, and as a result, a sizable number of them either dropped off the console scene or lost some of their old magic. Even SCEJ had troubles of their own, looking at their paltry and mediocre output this gen.
Even amidst all those troubles... one can technically argue that SCEJ had no real need to buy any Japanese development partner at this point of time. The bigger devs are out of reach, some of the smaller ones would not be viable investments due to them already shifting away from consoles... and the rest are pretty much Playstation exclusive devs already. ( due to appealing to the niche local market )
I think it would make a lot more sense for SCEJ to make a concentrated effort to establish one or two more internal teams within Japan Studio. They already have a lot of development talent there that's very scalable, with experience in multiple genres.