FREDERICK: Once a nerve fiber is severed, there is no way in heaven or on earth to regenerate life back into it.
Wrong! We later see how the doctor's work is able to do the impossible. Actually, scientists are working like mad (no pun intended) to regenerate spinal cord nerves. They've had some success in rats.
FREDERICK: Dead is dead!
Wrong again! The doctor later proves that death is only temporary.
FREDERICK: I am not interested in death! The only thing that concerns me is the preservation of life!
Ah, a true physician. But if this is true, how does reanimating a dead body with someone else's brain fall into the category of "preservation of life?" In Frederick's defense, he said this before he became obsessed with his grandfather's work.
FREDERICK: Just think. A dead brain ready to live again in a new body. […] Throw the main switch!
Perhaps the doctor's experiment isn't about the body at all, but about the brain. At this point, he thinks the brain is that of Hans Delbruck, and he's hoping to give this honorable brain eternal life. This is also the point of all that mind-uploading stuff. With the same brain (or simulated consciousness), we're still ourselves, regardless of the body.
FREDERICK: Life! Life do you hear me? Give my creation LIFE!
From the doctor's word choice, it seems that his experiments are less about bringing someone else back to life for their sake, and more for "his creation" so he can get credit for it.
FREDERICK: Alive! It's alive! It's alive!
This is a classic line from the original movie, but bringing the dead back to life will always be exciting, at least until it's commonplace in everyday life.
FREDERICK: What I have to offer you may possibly be the gateway to immortality.
If this is the gateway to immortality, where does the experiment go from here, were it to continue? How could this procedure be modified to give a normal person eternal life, or would they always have to put their brain into another person's body? All this talk about immortality; if it weren't in a Mel Brooks movie, would have serious religious overtones—resurrection, eternal life through Christ, the nature of the soul. Mary Shelley may have been thinking about this, but we doubt it was on Mel's mind.