It's not a matter of home consoles not mattering in Japan anymore, like it's a definite conclusion, it's not necessarily true.
Improving the graphics isn't sufficient for consoles anymore, so people moved to portables because they're more convenient. And this trend is going to catch up to the rest of the world, mark my words.
If Sony had come up with a concept that attracted people to the console experience, they'd buy consoles again. And by that I don't mean secondary experimental shit, like PSVR and Move. Instead the basic console has to present intrinsic value for today's times. I hate to break to you, but most people don't care as much about the visuals anymore. You'll see next generation, take a seat.
The answer isn't just making a handheld and expecting to break Nintendo's turf from the Game & Watch to today, believe me that is hopeless. They can however re-invent the value of their home consoles and return to huge popularity.
I KNOW they are not gonna do that however. The global success of the PS4 will blind them. They're gonna take a tumble again for the next cycle. But look no further than the PS4Pro, people don't care. It's not gonna work.