When making a river value decision based on what hands could call you, it's important not to take specific opponent out of the equation. I agree with Fnord. It's not what range of losing hands will call you. It's what range of losing hands THIS opponent will call you with.

As the opponent range opens up, so does the expected value of the correct decision to bet here. So I gather. You'll see what I mean when 77 calls a river value bet with 3 overs on the board, because he's loose and you didn't get extremely pushy.