well if you have 3 guys that are the wizards that can't be sacked, hire 3 contractors for a year long project to document things sufficiently that these guys become more sackable. Contractors of course have to be highly experienced as well, and well-remunerated, so maybe management doesn't want to go down that expensive avenue, which means it's a management problem.
Not to mention the years where they did not comment, help, document or whatever, and became uncontrollable sounds like a management problem over those years.
Your first suggestion might work, but having been there I know obnoxious programmers can be very obstructive, and put up some major barriers to newbies. That said, I think yours is a good solution if it can be afforded.
Per your 2nd point about long-term management problems, no doubt of it at all, but sometimes the new (and sometimes good) management simply inherits what previous mismanagement left behind.
Also perhaps you underestimate the power that programmers in well-bedded-in positions have. They can outright ignore management orders - experience speaking.
The other side of the coin is that new hires can be very obstructive. 80% of new hires here are relentless politicians who snitch to management that they are "underappreciated" while making one idiotic suggestion after the next to show their relevance.
In reality they cannot do anything, so they scheme to get rid of people who can. During the battle no useful work is being done.
not that that doesn't happen, but in the context of the thread here, I imagine it wouldn't happen (or be more rare). You're needing to hire senior COBOL experts, and the 'senior' part is where the sort of behavior you're describing doesn't happen as much. Usually it's a mixture of confidence in your own ability, a dislike of politics, and an assurance that you can get work someplace else if/when you decide to leave.
The people that 'snitch' and feel 'under appreciated', and so on... my experience is they have trouble keeping work, are afraid of being found out, and do what they can to get rid of others who can recognize their lack of skills. I just don't think you'd find as many of those getting hired in the context of the needs of this thread.
I'd like to back up what you say because I wasn't clear enough in my first post - I've known a few people like that, a very few. Most are good and do their best. Despite the impression, idiots as I described are very much a minority.
Maybe if job security were a real thing, those guys wouldn't create obstructions to guarantee it. If employees don't get loyalty from management, why should the street go one way?
The problem is that management doesn't care about the human cost of their decisions and it causes technical problems.
In the real world this approach will most likely lead to getting 6 indestructible wizards instead of 3. Magical staff that needs to be supported by wizards should be shipped to Hogwarts and replaced
Not to mention the years where they did not comment, help, document or whatever, and became uncontrollable sounds like a management problem over those years.