I don't understand the rationality behind this. Why have (nearly) impossible to reach ratings? I mean, by this logic, the most talented people will only ever be a 4, yes? So make 4 the limit. You just came off of saying you'd never make an offer to an 80K candidate who thinks they're worth 100K[0]. Yet, you're telling your very best that they'll never be _the_ very best because basically nobody is a "five".
[0] It's what you were basically responding to.
Edit: for clarification.