IME not so in the real world, you'll have accidental state in your distributed system outside the DBs. You'll have some stuff that actually always runs in one A

IME not so in the real world, you'll have accidental state in your distributed system outside the DBs. You'll have some stuff that actually always runs in one AZ in normal circumstances and your integration partner in another org has whitelisted only its IP. Etc etc, everything that is untested will find ways to conspire to rot. Especially if you haven't learned by seeing these bugs so you can avoid them. Also you won't have a clear experience and understanding of what happens in the failover and you'll won't know to avoid failover breaking mistakes in your VPC configs, security groups, frontend-backend shared state etc. (And by "you" I mean "your dev team", it's not enough that one guy gets it). Also^2 if you read the news about all the outages it's very common for failover systems to fail generally, not just AWS - the general engineering wisdom is: always test your failovers. And there's no substitute for end-to-end testing it, instead of individually testing each layer/module. (Bad: "we can skip testing db failover", good: "let's test that the whole system works when there's a az failure")