Cats are hunters by nature, and declawing is the equivalent of taking away their hunting capabilities which they need in order to survive.
Domesticated cats may not need to hunt for food, but there have been many instances where cats have been abandoned or became lost, and did not have the ability to hunt and survive.
My daughter took in a declawed cat after a friend said she could not take care of the cat. My daughter knew that the cat could not survive on the streets.
(BTW, my daughter is a veterinarian.)
Our cat has been declawed for nine years. He still “sharpens” his lost claws. He still hunts constantly. He still catches chipmunks. When a larger animal threatens him he moves out of that area as if fired from a cannon.