I've been both the junior and the senior in that kind of mentoring/millstoning situation myself.
As far as I can tell, there are only two cases when mentoring turns into millstoning:
- The junior is uninterested or incapable of learning from the senior
- The senior is uninterested or incapable of mentoring the junior
I've met both types, and they cannot be helped. Such a junior is not employable in that job (so only employable if they have sufficient skill and experience to do some other job), and the senior can only be employed for tasks where they are easily replaceable.
Either one of them can be the proverbial millstone. It is noteworthy that seniors who refuse to mentor juniors (typically because juniors will eventually take their jobs, and they want to maximize job security) are
easier to replace than the ones who are able to mentor juniors.
(A third case is also possible: both are interested and capable, but their personalities clash too much for it to work. While social details can be overcome, things like work ethic and tool use and organization can be showstoppers. This case is for a capable manager to resolve.)
Using a dead weight as a social tactic to keep a senior stuck at a certain task is detrimental to the company. Of course, that does not mean that sociopathic middle managers won't do it; but it does mean that when a Cxx-level executive does it, the owners of the company should be aware of the damage that does to the operations of the company. In many legislations, contracts between companies that keep an employee hobbled like that are quite illegal, too.
The reason is that a company does not pay its employees to be there; it pays for the results of employees work. Hobbling that up, a company is only making sure they extract less useful work from that employee than they otherwise could.
At the very thin spike at the top of the expertise hierarchy, we have people who may be employed simply because their knowhow is such that having them working for a competitor may be catastrophic to the company. The solution there is not social games either, but simply benefits, cold hard cash and well-written agreements both the company and the expert can agree to. A typical benefit in the software engineering world is paying them for 100% of their time, but letting them work on their own (often required to be free/libre projects, to avoid competing with the company, and specific subfields/software targets ruled out) for 10%-50% of the time.
(Because the companies so often turn out to greatly benefit from such work nevertheless, it is becoming more and more common for companies to offer that extra time for use in ones own projects, for even "lesser" experts in their software fields.)
As I am not an EE, I do not know whether similar practices exist in the electronics design field – YET. For certain, the amount of dedicated chips and even SMPS controllers is such that if I was the owner of such a company, I for sure would like my top designers to spend at least 10% of their work time on average experimenting with new chips and designs, just to make it more likely that they'll discover new, cheaper, more effective ways of using available stuff in the products the company designs. (There is also historical precedent across industries showing this is worth the expense for the company on the average.)
Of course, nowadays the problem is that if an approach makes 5× the profit in a year, but there is an alternate approach that provides 1× the profit in this quarter, the this-quarter solution is always selected. Very few care about long term, and instead live and die quarter-by-quarter.)
In Faringdon's case, I'd take a hard, objective look at the situation, and follow the money. It is extremely unlikely any corporate mill-stoning is taking place. If it indeed is happening, then it is much more likely that someone is defrauding the company, or making illegal agreements; similar to anticompetitive agreements among companies wrt. employee hiring practices. In all western countries, including the UK, there are government agencies who would be very, very interesting to hear the details if such agreements actually have been made. (This obviously includes the case where Faringdon is just reading the situation wrong; for example, misunderstanding a minor personality clash that can be overcome, as something more significant.)
In the real world, especially here in the Finland, there is basically never such an agreement, and the junior just turns out to be a (close) relative of (a close friend of) someone higher up in the company hierarchy. Then, anonymously divulging the situation to owners/shareholders (if Cxx-level officers are involved), or to Cxx-level (if it is just middle-management that is using the company as their personal cash-cow), carefully but neutrally documenting the situation and especially the loss of productivity and the hit it will do on the company bottom line, is the best course of action in my opinion.
(Others do recommend just taking your leave as fast as you can, but personally, I do get a kick out of making such exploiters essentially unemployable. It brings me joy, knowing that parasites failed to prosper. Others do volunteer work, I like to make life difficult for those who exploit others.)