The A6600 sensor first appeared on the A6300, actually, not the A6000, so it's "only" three years old. That said, they've bumped into the upper limits of image quality; the newer 26MP sensor (which you'll find in the latest Fuji cameras) actually performs worse in low light than the preceding 24mp sensors.

>Imagine how many iPhones Apple would sell if the sensor were not much changed from the 6s and the camera software not changed at all.

Honestly, very little of the advances in cell phone cameras since the 6s have come from sensors; it's all computational. We're up against the brick wall that is photon noise.

Finally, the A6600 is very much not a flagship. The A9 II is the flagship, at three times the price ($4500).