i believe the automatic gearbox being seen as 'fragile' as compared to the manual gearbox has more to do with hose much torque is applied at any given rpm.
with an auto box, you could, theoretically, put the pedal to the floor at 2k rpm and the gearbox would stay in whatever gear you are in, putting the maximum amount of torque that it can through the gearbox to the wheels.
with a manual gearbox, you have the option of dropping a gear (downshifting) before putting the pedal on the floor, raising your rpm and applying less torque at the lower rpm range.
with most automatic transmissions, you would not have the choice to downshift before applying full power, thus the full engine power is transferred through the gearbox throughout the entire rpm range.
i am sure i am missing some points... did that make any sense? (if not, pls excuse me as i have been sick these past few days)