I wouldn't say a TC adds "many" gears, more like only 1, a takeoff gear. Mostly because if an OEM programs it so as to use slippage, it's going to generate gobs of heat and that'll lead to problems of its own. Once you hit stall speed & exceed it, torque multiplication drops off dramatically (as does heat generation).
I'm a bit dubious about my car "safely" towing 2k vs my wife's car doing less. My car has like 10" discs and drums while her car has larger discs all around, then toss in 14" tires vs 16". Yeah the trailer ought to have brakes, I know, but I swear, my car squats more with loading than her car does. Her car does have a pretty deep takeoff ratio, not sure the clutch would be that abused. Just seems odd that my car is rated for more. [In the end I know the OEM's would rather me buy a truck.]
Lastly those OTR trucks have "many" gear ratios to deal with the limited rpm span of the engines. Correct me if I'm wrong but I don't think many of those engines like being pulled below 1k yet have an 1,800 rpm limit. Running at either extreme might not be best for the engine (nor driving). Many ratios so as to keep the engine around peak torque where it can pound out the power, hour after hour. [I've heard of lugging some down to 400 rpm and I'm sure Thermodynes had something like 1,200 rpm limit: I'm thinking just general case.]