High churn is a high bus factor, like by definition of a bus factor. Bus factor => people had kids, retried, got a better offer, got fired, or were incapacitated (the proverbial bus) on a moments notice, and thus are abruptly unavailable to maintain your systems.
Churn is not a mechanism in itself to reduce the bus factor. It doesn't encourage people to mitigate the bus factor, because if you're going to be pushed out anyway, why bother building something sustainable?
I could see the argument for a probabilistic approach in that the company continues to function with critical systems being black boxes that teams have so far successfully scrambled to replace (with varying degrees of success), kind of like a more permanent Chaos Monkey for an entire system instead of cells within an system.
However, that does seem like it would waste a bunch of time, money, and opportunity on churn to deliver what you already have instead of using experience to improve/replace the existing systems.
The other bit is it's a form of resiliency. High churn improves the bus factor. I respect Amazon for building a system that's resilient like this. That's not saying it's a good system to work in, but I admire that they built a system that can operate like this.