No.
B12 deficiency can cause irreversible, permanent, nerve damage. The common story of vegan babies dying is from them not having b12 from invitro, or not being supplemented after birth.
B12 stays in your system for years without supplementation, however when it runs out, you can soon be in trouble. Vegans would not survive long enough for any change to occur, and would reverse so much progress we have long fought for in making plant based diets mainstream