I wasn't trying to argue that the optimal amount to spend on employee turnover is infinite, but I do think most companies dedicate less than the rational/optimal amount of effort on it.
Assume that the company loses 10 employees and loses $1.57M as a result.
This means that it would be better off if it spent less than $157k on each employee, on the average, to prevent the loss.
$157k is a yearly salary of a senior developer, or maybe the entire yearly cost of a senior employee outside the hotbeds like SV or NYC.
We can cautiously suppose that a company could retain the employees with measures less drastic than paying them twice as much for a year. OTOH paying them +20% for 5 years would be the same. So, if they can keep hiring cheaper developers, even while losing productivity due to onboarding issues, maybe they can actually save money!
Recruiting costs are typically worth several years of 10%+ raises.