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.

If you're hiring people like this, I don't think old code bases are your biggest problem.

If you hire people like this you need to involve HR in better selecting who the org hires.