The SSD market has been anything but stagnant for the past three years. That's a ridiculous assertion for you to be making - three years ago, the SSD market was still immature and not at all ready to be mainstream. The first halfway-decent SSD (Intel X25-M) is only about four years old, and it's nowhere near competitive with current SSDs for price, capacity, or performance.
You seem to be under the false impression that the NAND is the most important part of an SSD. It's not. From an engineering perspective, it's the least important component - it just happens to be the primary reason for cost scaling with capacity. The controller and it's firmware are far more complicated, and make all the difference for performance. Those components have made a lot of progress in the past three years. And even the NAND has advanced, just not exponentially, because while density is inversely related to unit cost, it is also inversely related to durability, and the drives 3 years ago weren't designed with excessive longevity requirements.
You seem to be under the false impression that the NAND is the most important part of an SSD. It's not. From an engineering perspective, it's the least important component - it just happens to be the primary reason for cost scaling with capacity. The controller and it's firmware are far more complicated, and make all the difference for performance. Those components have made a lot of progress in the past three years. And even the NAND has advanced, just not exponentially, because while density is inversely related to unit cost, it is also inversely related to durability, and the drives 3 years ago weren't designed with excessive longevity requirements.