Wrong every animal instinctively when sick or injured fasts on water till they are healed. It's been observed time and time again that when a deer has broken its leg it will go to the nearest watering hole or river and just drink water till its healed. there is new science coming out finally that fasting has amazing benefits here are some links for anyone interested
https://news.usc.edu/63669/fasting-triggers-stem-cell-regeneration-of-damaged-old-immune-system/
I suggest you do some research on the topic as there is heaps of information out there.
As much as water is necessary for your body. It doesn't provide the required nutrients to sustain a person endlessly. We can go on without eating anything for 2-3 weeks, maybe more, but that depends on every individual and its body mass.
Without eating for a period of time, we will start to use the fat we have saved in our body, after that has been depleted, the body "eats" itself. Using the proteins of muscle mass to gain energy and lastly dispensable organs. The brain and heart needs energy to continue their work.
That is why models have almost no muscle mass.
And your example with animals, how long do you think can an animal live without water? Except camels of course. And to add to that, an animal has more fat reserves than humans, they can go on without food for a much longer time.
Also for a broken leg to heal, it takes about 4 to 6 weeks, if it isn't an open fracture, the bones unable to grow straight without something to hold it in place.
Edit: Ah one more thing, what would you do if you are in the middle of the forest with a broken leg? You would fix it, alright. But how will you survive? You can probably survive without food but without water for 3 days? That would be difficult
Proof is in the doing. the video I shared shares heaps of information if you really want to seek answers. Fasting saved my life. don't run faster then you are able to and start on shorter fasts and work your way up. keep in touch with a doctor to moniter your levels if you want to exceed 10 days water only. Your body doesn't consume itself unless you are already severly malnourished,But it does break down inferior matter and consumes that as proteins such as cancers tumours cysts skin tags virus's. it is always encouraged that during a fast when you start genuinely feeling hunger you should eat again.
Without nutrients how will the body be able to produce antibodies? White blood cells to defend you against viruses?
Anyway, about cancer cells. The body will not consume it but slow down the growth of those cells, since there are no nutrients to support it. That way you can fight it easier.
But nevertheless it is dangerous to fast for such a long time and can actually have long lasting aftereffects.
And you said it yourself, if you want to exceed 10 days you have to keep in touch with a doctor. Would you call that healthy?