I see this line of argument. But the thing is - biting is something that is culturally unacceptable. Whether right or not, biting will be seen as worse than a punch or a bad tackle. And he has prior history. At a certain point these things have to be accounted for.
Keane's horror tackle should have resulted in a long, long ban. It's worse as an isolated incident that this, objectively. However, just like spitting is seen as terrible, so is biting. If Suarez cannot abide by recognized cultural norms, even after repeatedly being punished for failing to abide by those norms, then one has to come to the conclusion that he's not fit to be on a football pitch.
Actions cannot be taken in isolation when one has an extensive prior history of breaking the decency and normalcy of the football pitch. FIFA should take into account that Suarez has been banned for biting before, and banned for racism, and every other action that he has been seen to commit. This isn't about this bite. This is about the fact that he has proven incapable of controlling his actions. Or worse still, he is capable of controlling himself, and chooses to recurrently act in this manner.