I actually don't see any issue that isn't simply a matter of money. Everything they list that's an issue seems to revolve around the fact that the employees with domain knowledge for the system have left the company. Reach out to them and hire them on as contractors. If they left under bad terms because the business was a bunch of dicks, expect to pay 10x market rate. If this is truly "fix this or the business is out…
Is there another industry where a few employees with "domain knowledge" can sink a company? Asking because, while, yes, throwing money at the problem might be a solution, I think the obvious bigger issue is how does one allow themselves to get into this situation? Or rather, how does one avoid getting into this situation. Because it seems to me that the software industry of late has become so mired in esoteric layers…
Mentorship and documentation.
You need to set aside time and resources for these two things, and make contributing to them worth something in the performance review.
If all you do is count JIRA tickets closed, you'll forever end up in this situation.