Backing up across two different regions is possible for any provider with two "regions" but requires either doubling your storage footprint or accepting a latency hit because you have to make a roundtrip from Fremont to Denver.

The neat thing about AWS' AZ architecture is that it's a sweet spot in the middle. They're far enough apart for good isolation, which provides durability and availability, but close enough that the network round trip time is negligible compared to the disk seek.

Re: bit rot, I mean the frequency of events. If you've got a few disks, you may see one flip every couple years. They happen frequently enough in S3 that you can have expectations about the arrival rate and alarm when that deviates from expectations.