I can't speak to how the demographics of the back-fill would turn out, though I'd agree what you say seems likely. However I will say that that Junior engineer is also likely getting a pay rise from his old job (or perhaps just entered the work force).
As to your second point regarding the cold calls if the cold calls do not have some benefit for the one called then it wouldn't matter if other companies were making them (as why would the employee leave). The fact that these companies colluded to avoid them speaks to the fact that threatening to quit vs being poached does not have the same benefit for the employees(or that not all employees have the tendency to act as mercenarily as perhaps they should).
As to your second point regarding the cold calls if the cold calls do not have some benefit for the one called then it wouldn't matter if other companies were making them (as why would the employee leave). The fact that these companies colluded to avoid them speaks to the fact that threatening to quit vs being poached does not have the same benefit for the employees(or that not all employees have the tendency to act as mercenarily as perhaps they should).