If your dog is losing it's teeth and has no teeth left then it still can eat and survive just fine.
When your dog loses it's teeth you'll have to feed it some softer dog food instead of hard dry dog food or soften up the dry dog food with some water or broth.
You can also blend up vegetables or other meats such as chicken or hamburger and feed your dog that food instead.
Vegetables that humans eat are very healthy for the dog so if you mix them up to a consistency of baby food then the dog will be able to eat it without teeth.
The dog will be able to eat soft food that you buy in a can but you should also mix up vegetables in a blender to feed them that as well.
The dog can also eat some cooked rice when they do not have teeth.