In an interview Sony gave to AV Watch recently, the company admitted it's going to "gradually end development and production" of recordable Blu-rays and other optical disc...
It's getting harder and harder to make things smaller, but they are making things thinner now, which means they can layer them, thus increasing density.
Again, writable Blu-ray is a slightly different format from what you would buy with a movie on it. They are not putting Criterion movies on the same discs you could burn them to in a drive.