I'm not with Google but the SRE book has a whole chapter on this (Ch. 3 Embracing Risk). For most services you don't need reliability quite as high as you'd think. There's a background error rate caused by networks and devices you can't control, and once your service is more reliable than the background rate then you should shift focus to feature development instead.
The book severely simplifies this concept and borders on error when talking about the general reliability of the network vs your service. It's an interesting point to consider though.
