You have to realize that a company is always optimizing for efficiency and salaries are no different.

Giving out promotions when people are already working at the level they'd be promoted to is simply a waste of money.

This is the author's biggest mistake. If you voluntarily work on tasks above your pay grade you are signaling to the company that you don't need a promotion.

There isn't a single optimization. Define efficiency. Define over what time frame.

The problem the OP faced is that YouTube is optimizing under a short time frame and under the belief that employees are fungible. The latter being a common problem with big orgs, thinking there is no value to institutional knowledge. Yet in reality that is often extremely important