The only issue with only eating animal priducts is that the Paleolithic people had a complete diet with no deficiencie.  Modern Paleo people do not have the same situation.  Paleo people did eat some non animal foods but even if they didn't they would have a complete diet.  This is becasue they ate organs, blood, bone marrow, bones, extra fat found around the organs(kidneys, intestines) and they ate the stomach and intestinal contents of their prey.  This is partially digested vegatation.  The muscle meat was actually eaten after all these things.  Carnivorous animals take the same approach.  So where are you going to get blood, one of the most nutritious parts of an animal?  You can eat fermented veggies as a replacement for the intestinal contents.  Our ancestors got vitamin C from eating adrenal glands, B12, B6, folate and kinds of other nutrients from eating liver, minerals from bones, all kinds of nutrients from blood etc.  I see some people today thinking that they can just eat muscle meat and be healthy.