Redundancy is only helpful if the redundant systems are actually functional.
It's a fact of life that when dealing with complex, tightly coupled systems with multiple interactions between subsystems that you will routinely see accidents caused by improbable combinations of failures.
AWS could create some machines at a lower cost and lower availability, just something that if goes down doesn't affect you much, or one-off usages.
I'm not sure how migrating machines between nodes happens in S3 or if it's easy to do it (maybe with some downtime)
It's the same as advice to routinely replace your live data from backups. It's not a real backup until you've tested that you can recover from it.
eg. If you have a test that is 99% accurate and a treatment that harms 1% of the patients and you do this screening of a million people - how common does the disease have to be before you cure more people than you kill ?
Note how in the case of backup power, "properly tested" doesn't mean 'Does the generator turn on? Are we getting electricity from it? Ok, pass!'. It means running the backup generator in a way that is consistent with what you would expect in an actual power failure - i.e., for more than just a few minutes.
Same thing with storage backup. Checking your backups isn't just 'was a backup file/image created?', it means _actually trying to recover your systems from those backup files_.
AWS would have found this, and been able to fix it in a timely fashion, if they did the same (the genset lasted for 10 minutes under load before failing).
Diesel is great food. For bacteria, that is. So it's treated, but you've still got to stir it to keep gunk from settling, you've got to rotate it (so you burn through your stock every so often), you've got to filter it. And stages of all of that can go wrong.
I recall a cruise on a twin V12 turbodiesel powered ship (hey, we've got full redundancy!) in which both engines failed. Cause? Goopy fuel and clogged filters (she spent a lot of time in port). This happened a couple of hours into cruise, fortunately on inland waterways, not open seas, shallow enough water to anchor, and numerous skiffs with which we could head to shore and find replacement parts.
More recently, an colo site I know of was hit by a similar outage: utility power went out, generators kicked in, but a transfer switch failed to activate properly. Colo dumped.
Second time it was the fire detection system which inadvertantly activated. One of its activation modes is to deenergize the datacenter (if you've got a problem with too much energy liberation, not dumping more energy into the system can be a useful feature). Again, full colo dump. And APCs will only buy you enough time for a clean shutdown. If you've got them at all.
But, yes: exercise your backups.