This is highly debatable, although protein based diet is highly encouraged, come to think about it for real.
Do you think meat was 100% available for those couple millions of years you mentioned? Perfect abundance for that whole period? Also no individual would have ever decided to eat a ripe and delicious mango ready to be consumed? Bananas? Consistently turning down all fruits?
Highly unlikely. There are cycles and seasons, variety in diet makes the most sense. Meat based for sure, but throw in some fruits, berries, honey, nuts, etc for those hard times or just to spicy life up.