Earlier quoted context omitted.
What's the justification for an attrition target? It seems such a weird thing to want your staff to leave.
If you have a hand of cards you want to optimize it makes sense to keep discarding and redrawing your worst cards. At the beginning your worst cards are probably below average, so any new replacement is statistically likely to be better. But even when you reach the point where all your cards are above average it can make sense to keep redrawing a small percentage, since that's the only way to get more aces. Of course…
This is where your cards analogy falls through, in a deck of cards, you know that there are better cards you don't have in your hand, you know exactly what your Jack is worth in absolute terms.
At Amazon, you can have a Royal Flush, and because of stack ranking decide to discard your 10, and if you know your poker, that makes you go from the best hand to one of the worse.
The issue is you wouldn't know your 10 at Amazon was a great engineer and a key asset to your team, and that all replacement candidates are going to be worse, except now you've lost precious time, ramp up, and moral, as well as a great employee.
For a smaller company, it might make sense as you're building a core team and might need to go through a few people before you find a set of good ones, but where Amazon is now, at its size, over time, it has lost engineers that in industry wide averages were above it, and it has created a reputation that hurts it from attracting them back or other engineer's above the industry's average.
Due to this, Amazon is becoming a second pick. Good engineers will pick Netflix, Google, Microsoft, and even sometimes Facebook (due to their higher comp) over Amazon. And since all these companies hiring process are the same, it's common that someone who can pass the Amazon interview can pass one of those other ones as well. In fact, almost 70% of everyone I've seen be released from a PIP went to work at one of the other big tech companies.
The other big problem is that the stack ranking works like American reality TV competitions. It doesn't matter if you've excelled year over year, if you have one bad year, where for whatever reason you get identified as a low performer, you'll get PIPed. In my experience that's often contextual, a new manager or you look like a bad performer because the business and leadership couldn't figure out good projects for you to work on and own that year, or they themselves couldn't get their act together and you're a scape goat. Other people on your team might have performed better simply out of luck of having been assigned better projects.
Amazon used to get away with simply finding the best talent, but in my experience there, that's now backfiring, and instead of growing the best talent, their quest to find it is turning into a system that recruits more and more mediocre talent as time goes on.