Immutability doesn't really solve everything. It provides a cleaner path for retries for writes, but still doesn't handle situations where reads fail.

I think the conclusion in the article ("don't do fallback") is misguided. Fallback code is sketchy, but sometimes it is worth it to take the time to write well-audited, well-tested fallback code to ensure a system which has high availability requirements can survive dependencies which are less reliable.