It really spends on how reliable you need to be. Don’t forget you get downtime from both AWS and your own issues so even 4 9’s is off the table with pure AWS. If you need to be more reliable than AWS you need to run a hybrid inside and outside of AWS which means most of the advantages of running on AWS goes away.
Many business claim 4 9 SLAs on AWS, but that doesn’t mean they actually provide it. It’s simply a question of what the penalties of failing to reach their SLA is.
