Well you can also just count current employees' tenure. That fixes the problem and is qualitatively consistent IMO.

Then you get into the field of survival analysis, where you get methods that can handle this problem (called right censoring IIRC).

https://en.m.wikipedia.org/wiki/Kaplan%E2%80%93Meier_estimat...

Exactly. If half your workforce is happy and has been there 20 years, that’s entirely non reflected in such a figure. There must be a better metric.

What do you mean? Every employer I've worked for had data for the starting date of their employees, when someone leaves you have an end date of employment. It's a pretty simple metric to calculate.