Would Google's bottom line drop by ($500m - $devSalary) if that developer were to quit this year?
Of course not! In most cases Google's bottom line wouldn't even notice if Google failed to replace them, but if their job was really important and they did have to replace them, they still wouldn't notice that it was someone else and not that one dev.
This suggests that the dev isn't actually what's making Google $500m/employee, it's the systems that make up Google that are making Google $500m/employee. Any one employee doesn't actually contribute nearly that much to the functioning of the system, otherwise every company would make $500m per developer.
So the only sense in which Google "should" pay more isn't a practical one (clearly they pay enough to fill their roles) or a justice one (they're not stealing $500m in value from each employee if their bottom wouldn't much notice any single employee's absence)—it's some sort of argument about an ethereal sense of fairness that says that by merely participating in a system you're entitled to the average per-person output of that system. Which is definitely an ethic, but it's not one I can get behind.