i read interview with one of the production managers for Toyota who said that at some point, you dont keep saving more money when you are producing more.

If i had to bet, it would probably be cost of engineering new powertrain into highlander package.

 

Toyota is greatly increasing 6-speed production over the next year and a half, so expect to see the 6-speed on the Highlander and other Toyota vehicles soon, models that don't already have the 6-speed.