[Noumenon72]

The model might work better if you assume an increasing median among the population of candidates over time. This could be true if:

* Amazon's stock price and profits made it a more desirable place to work over time

* The increasing supply of computer science graduates improved the hiring pool

Another thing that could happen is the most skilled x% of each team leaves over time for better positions, so that the median slips back. Then raising the bar each time only keeps it in the same place.

Or candidates could be rated on multiple dimensions, so while you can't find someone uniformly better than your median employee, you can hire one person to raise the bar on build systems, one on microservices, and so on.