Imagine nothing dies then after one year what will be the situation on our earth?
The oxygen might be consumed and at one point all people will die at the same time.
The earth which we are living on is well balanced, assume the lions do not eat meat will the lions be strong?
Scientist proved that for a healthy body you need proteins, and the best natural source for proteins is meat.
I remember a story of an Indian man who was sick and went to see a doctor told him go have some proteins and come after two weeks, the man came but there was no improvement, and the doctor asked did you take the proteins, the man said no, the doctor said why? The man said I am Vagetrian, the doctor said sorry I can not help you.
Nowadays there might be alternatives for meat source.