On the other hand, how many times in your career have you stayed at a company more than 9 years?

Doesn't matter. It is a lopsided relationship. As an employee, it is massive portion of your life. From the employer perspective you are a resource, and can be replaced (not making a moral judgement, just saying that you can...). So the fact that someone can leave after 2 years is less impactful on the company than a company that forces you to leave after X years.

If on the other hand you are just saying "well it is unlikely you will get to 9 years" the problem with that thinking is that the closer you get to 9 years and the more you have invested the more likely you will want to stay for 9 and being forced out for no reason is silly. It is a job, not the presidency.

> you are a resource, and can be replaced

This is exactly the disconnect between people's salary expectations and their actual salary. It's not really based on how valuable your work is, but how easily replaced you are.

Yes and to be completely fair, the benefit for an employee is to get to focus on one specialty rather than run a business. And have a known paycheck. And get experience while being paid to do so.

As you get older and have other responsibilities that life gives you, constant job hopping gets old too.

Honestly as long as a company has enough variety internally I (slightly) prefer staying with until they "go bad".

Now I also prefer startups and my carrier isn't yet that old (like ~8 years) so never is still the answer.

Through companies being often way less likely to give a trusted long term employee a payrise or trust them with more responsibilities then they are to do so with a new hired employee they have hard times to properly judge the skill of is still an (absurd but not IT specific) issue.