They're sort of getting what they want. While it's true that a lot of people are leaving, their replacements are not living up to expectations. I'm sure they love it when a 15 SPH senior courier quits, but when the new hire can only hit 8 SPH and will probably never get much better, how much are they really gaining? However, they are still expecting 15 SPH from the newbie, who will soon be progressively disciplined out the door.
I totally agree that they are creating a company where it's 3-5 years and then out the door...at least that seems to be the plan. But at my station, which is probably fairly representative of most, a very high percentage of the new hires quit within a few weeks. We have 3 who have stuck around for over a year, and I know 2 of them are actively looking for something else.