My gut feeling is that if your only AZ goes down (or all your AZs simultaneously), you're going to lose data period because your producers are now all stuck, your APIs are unavailable, etc. Whether the data loss begins at the exact moment power failed or a couple minutes before doesn't matter, vs. the additional cost to fsync constantly.
I mean it's good to know all the failure modes, but at the end of the day it's also good to know how much handling them will cost, and it's often not worth it.
