After a team member left and a new maintainer had to step in to maintain an unfamiliar code base, our new EM decided that the bus factor must be raised across all projects. So every quarter we shifted responsibilities to new projects and had someone else take over ownership - the idea being that the previous person would still support and we'd not be in trouble in case someone left. That's not what happened though.
It didn't really matter if the old owner was around to support. Productivity plummeted with every project. It was as if every owner of every project had left the team every quarter.
Now I'm more convinced than ever that it's better to pay the price of an owner leaving if and when it happens, rather than try to prevent or mitigate that risk up front. The productivity costs are far too high for most software products.