[edmundhuber]

If a system is not documented in terms of what it does, how it does it, and common remediations for when things go wrong, then it's a fragile system depending on one person to keep it going.

You need to get (particularly, senior) engineers to buy into the mindset of documenting everything and making the documentation the first place to look (not after all else fails). Leads should hold be held accountable for their docs meeting some standard of "this is a useful doc".

If I were in your shoes I'd get senior engineers in a room and be honest with them about the situation. You're going to need them to do work that they might not be naturally inclined to do, or work that might not seem like a "productive" use of time. You might want to get them to agree on what a good doc looks like (the standard), and what things need to be documented. Maybe take a whole day to do this, with snacks and coffee. Good luck!