I doubt it, often headcount back-fill puts in a more junior person. So post poaching, you might see the company hire a replacement outside engineer at lower salary than the person he replaced.
Also, again, this is only about no-cold-calling. If someone truly thought they were hot stuff, and wanted to cash in, all they need do is threaten to quit, seek out an offer themselves, and they could gain your "fairer" distribution.
In fact, it was not at all uncommon to people to threaten to leave and get big counter-offers to stay in recent years, especially higher level employees.
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).
Also, again, this is only about no-cold-calling. If someone truly thought they were hot stuff, and wanted to cash in, all they need do is threaten to quit, seek out an offer themselves, and they could gain your "fairer" distribution.
In fact, it was not at all uncommon to people to threaten to leave and get big counter-offers to stay in recent years, especially higher level employees.