I own a 6.6l gas GM, they ain’t no slouch, towing max 9000lbs bumper pull 6000ft. I’ve owned two duramax’s and this holds it own, much better than my old 6.2l gas GM 8 speed, better low end torque.
Having a 10 speed is better for sure but I can guarantee this old fashion 6 speed will outlive that Ford 10 speed, GM specifically chose to use this proven tranny to avoid any first year issues, as seen with Ford’s 10 speed. Compare the pricing on the trucks and go from there, when I bought a year ago it’s was significantly cheaper to buy a 6.6 base truck over the 7.3.
GM is always behind the power in their vehicles, 6.0l was around for almost two decades while Ram owners were rebuilding 5.7l engines and Ford owners doing cam/phasers on their 5.4l. Don’t expect GM to do anything different I would bet this 6.6l is unchanged for many years and they will add a 10 speed soon.