I think that this is, to Andy's point, basically about simplicity. It's not that your business necessarily needs 11 9s of durability for continuity purposes, but it sure is nice that you never have to think about the durability of the storage layer (vs. even something like EBS where 5 9s of durability isn't quite enough to go from "improbable" to "impossible").

There are a lot of companies who their livelihood depends on their proprietary data, and loss of that data would be a company-ending-event. I'm not sure how the calculus works out exactly, but having additional backups and types of backups to reduce risk is probably one of the smaller business expenses one can pick up. Sending a couple TB of data to three+ cloud providers on top of your physical backups is in the tens of dollars per month.

Different people and organizations will have different needs, as indicated in the first sentence of my post. For some use cases one server is totally fine, but it's good to think through your use cases and understand how loss of availability or loss of data would impact you, and how much you're willing to pay to avoid that.

I'll note that data durability is a bit of a different concern than service availability. A service being down for some amount of time sucks, but it'll probably come back up at some point and life moves on. If data is lost completely, it's just gone. It's going to have to be re-created from other sources, generated fresh, or accepted as irreplaceable and lost forever.

Some use cases can tolerate losing some or all of the data. Many can't, so data durability tends to be a concern for non-trivial use cases.