I kind of feel like this is going to end up with the Ninomiya family still "falling," but Sayuri did enough to prepare herself and her family (and to avert the hardest parts of the fall) that they are able to go on with a relatively happy life. Like maybe the grandpa retires with inner peace (even if his outside reputation isn't as high as it was), her parents learn to make a more modest (but still good) living with their circumstances, and Ninomiya will have a good future ahead of her, with real friends and skills.
Still a happy ending overall. I mean, all of Japan is about to get upended with some hard times, if Sayuri can preserve a decent life for everyone around her, that's still a big win.