In all my years in the trucking industry I have never had a truck go down for transmission issues. It is usually clutches and compressors or AC brackets. I did see a guy tear up a differential by splitting the axle at 70 MPH. The only way not shifting without a clutch does damage it pulling it out of gear under a load. Road rangers fall out of gear just looking at them, Fuller Eaton’s aren’t much better but the Rockwell’s are pretty tight. I don't know any drivers with miles under them that use a clutch. Most semi tractors go 700,000 miles before you rebuild the top end and the lower end at 1,000,000. I have herd them going 2,000,000 miles but that is very rare. The rest of the rig is pretty beet by then