Companies are a quirky institution from a pipelining perspective. Schools are easy to model: you take in a new class every year, they mostly predictably level up, and they mostly predictably graduate, on schedule. Cradle-to-grave institutions are still *fairly* easy to model.
Participation in companies is not quite 100% voluntary, which is an interesting wrinkle, and can be a really sharp cultural divide between companies based on how large epsilon is. Liquid labor markets with exit as an achievable option are unfortunately not a law of nature.