Why do you say 6 speed parts last longer?
I guess the slightly wider teeth and chains may be better than a 9 speed but an 8 speed uses the same chain width.
6-speed hubs use a freewheel which means the hub bearings are further inboard and more likely to have a bent axle. The axle width is also narrower at 126mm which means there is not much if any advantage in wheel dish.
My guess is they used a 6-speed drivetrain because it was cheaper. I still see cheap bikes with 6-speed derailers so I'm guessing the system costs less than the more modern 8-speed drivetrains.
I'd rather have the hub gear with additional rim brakes.
Craig