The reason for the drills is the danger of failure of mechanical systems.
I don't disagree that it's a possiblility, but it's a VERY remote one. The planes system is very robust with an emergency mode for loss of electrical power and local manual control. All modes are tested before each dive. And, only a stern planes jam could cause loss of depth control.
Again, a possiblity, but very remote. Unfortunately, these things are usually cause by human error.