This works both ways.
The value of internal knowledge helps both parties, because as an employee you dont know if x other company is good to work at, or that you would like the responsibilities there, etc. The fact that lots of people stay with a lower salary in a job is a testament that it is often the employees that choose to pay that fee.
The company is frequently interviewing. So if the company finds a candidate that would work for less (minus the internal knowledge comission) it should fire you and replace you!
So the fact that "company is frequently interviewing" only shows how much demand there is and how hard it is to fill a position with the right person. Once you have the right people on board, they can ask a lot more than what they get, because replacing them can be significantly more time and resource/money consuming and then it's not even guaranteed that they will be as good as the ones you have right now.
This is basic free market, the side of it which has the upper hand right now needs to play its cards to max out wealth and every employee should be encouraged to do so, because it will overall only increase economic growth. It doesn't always has to come from business growth, if workers can gain significant wealth based on the same principle then they should, because it will only increase more spending and investment of those people into new areas again.
I loved your comment about "max out wealth" because that is the underlying truth to all of this--you get maybe 30 years to make as much money as possible and that's it!
This is of course for people that value what they do at work and want to make meaningful impact professionally in their lives. For other people family is much more important so it doesn't matter if they don't min/max the projects they work on.
That employees "pay that fee" doesn't indicate why. It also doesn't provide insight into whether it makes economic sense for them to do so.
This is a flawed philosophy. As a manager I have had a few critical pulling in higher salaries than my own. If you are a superviser who is artificially holding an employee back from reaching their full potential (seeking compensation that matches the demand person's talent in the open market) than you are doing your company (yourself) and that employee a major disservice.
Just because you are a manager doesn't mean you can do what your employee does (and vice versa). Artificially holding a sub-employee back is a sign of fear for one's own ineptitude.
EDIT: My main responsibility is to keep the team happy and productive. If this developer would leave the team our productivity would drop and we wouldn't achieve our goals we set for ourselves this year. It would have a direct impact on my bonus as well and it is in my highest interest to keep great people at every possible cost. If I think I don't earn enough then I will have that chat with my line manager, but I will never try to destroy my team based on some personal ego crap.
Which is better for the company? (A) Leader acts in such a way to make him or herself 150% effective, or (B) Leader acts in such a way as to make all of his or her direct reports 115% effective?
If a leader can't make their team more effective, then why have them in leadership?
(Edit: Essentially, what dustinmoris said!)
It is basically just a question of market forces. If a particular position is sufficiently specialised when the supervisor role is more generalised then you might expect to pay the specialty more than the generalised role - even though the generalised role is more senior.
Also, few people are truly indispensable. In all my time working, I've never seen a company fall apart from the loss of one important person. In fact, it usually takes a shorter time to cope than anyone anticipates.
I do agree with you, In the sense that as an employee, you're in great shape if you're seen at being essential, to some degree; I just don't think you're capturing the entire market dynamic.
Also don't believe any line manager or anyone else here posting that this is not a good strategy, because remember, every line manager is only an employee as well and has a line manager as well and they want to get paid a fair salary as well, so they either will do the same or they will have 100% understanding for it and if they don't then you know that the company has no interest in paying you a fair market value for your contribution.
This is the free market, companies love to play the game when it comes to having people let go (like SoundCloud), when it comes to taxes and when it comes to profit distribution and growth, so they should also expect to play the same game when it comes to negotiating employee salaries.