it is true all cattle are started on grass... but industrial beef is finished with corn. and non industrial beef is grass-FINISHED. it is a big difference. compare a steak from the two, it is night and day.
they only eat corn for a few months, yes, but during this few months at the feed lot they put on a massive amount of weight. this would eventual kill them indeed.
you contradict yourself when you say:
"ALL cattle are grass-fed up until the last 3 months or so of their life, and then corn-finished."
" there is no such thing as corn-fed/corn-finished beef. "