I agree with you that the trap bar is a piece of shit.
But the sagittal plane movement argument is not persuasive. "most muscle mass over useful range of motion" Having to stabilize the sagital plane will require, theoretically, the use of greater muscle mass. We dont recommend the Smith machine for bench press because we want 3 planes f instability and smith machine eliminates two. If deadlift eliminates (sagital) plane of instabuility and trap bar elinates none, doesnt that make trap bar superior? Are squats stable in the sagital plane? Horizontal? frontal? NO. But its still the ideal exercise.
Again I agree with you that its a a piece of shit exercise (the grip reason is perfect) but the sagital plane argument aint it.
The argument that trap bar is a standing leg press is a better argument - that it allows for sloppy spinal mechanics is a better argument. that it is irreproduicible is a good one. The grip argument is a good one. That it - unlike the DL - doesnt requires someone to learn how to brace their spine is a good one.
My 2 cents.