What does that mean, though?

If I'm storing data on a NAS, and I keep backups on a tape, a simple hardware failure that causes zero downtime on S3 might take what, hours to recover? Days?

If my database server dies and I need to boot a new one, how long will that take? If I'm on RDS, maybe five minutes. If it's bare metal and I need to install software and load my data into it, perhaps an hour or more.

Being able to recover from failure isn't a premature optimization. "The site is down and customers are angry" is an inevitability. If you can't handle failure modes in a timely manner, you aren't handling failure modes. That's not an optimization, that's table stakes.

It's not about five nines, it's about four nines or even three nines.