Given that fasting is always temporary, and that in one week of fasting you only consume 25% more proteins than an entire active day (remember you are supposed to rest while fasting), it's doubtful. Especially since fasting activate the production of growth hormones.
Not that you won't loose a little muscle. Of course you will on long fasts. But "all your muscle mass" is just plain silly.
Also it just doesn't happen with intermittent fasting: you tend to gain muscle.
2. Suffer malnutrition.
Malnutrition is a condition you get on the long run. Not on a few weeks on not eating a year. Besides, the improvement you get on your digestive systems after putting it to rest improve your ability to absorb nutriments after the fact, so if you follow a balanced nutrition after your fast, you should actually be better off.
And any fast minded person will tell you that what you eat after a fast is as much important as the fast itself. Once you empty the body, you gotta fill it with good things to allow it to rebuild clean what the autophagy removed.
3. Destroy your stomach lining due to too much acid building up.
Acid production stops after a few days because there is nothing to digest. The rare cases where people suffered from stomach lining were people with terrible eating habits that had acid problems already and went full throttle on fasting. Nobody knowledgeable about the practice would forget to tell you how important it is to prepare your body days or weeks before you fast. Especially if you are not used to it.
Have you ever read any study about fasting at all ?