Outside of the politics and trust portion, most of this should be well documented and not something that relies on a person being around. What's been tried, and why, should all have documentation around it - what worked, what didn't, how the decision was come to, what data points were considered, etc. All of that is valuable, and it should have all been documented along the way to help back up the decision making process to begin with.
When the time comes to make decisions in that same vein, people can review the previous process. Maybe they have new information to bring to the table. Maybe they didn't account for something that was considered last time. Maybe the "you" of whatever process is no longer with the company.
I'm in a similar position to you. I've made it a mission the past couple of years to try and eliminate myself as a single point of failure for this sort of information.
To the second point, just because the job posting isn't there doesn't mean the job doesn't exist. Companies might not be looking to necessarily hire a staff engineer specifically, but could see the value in one if the right candidate appears.