"Have you heard the phrase "You're not Google, you're not Facebook, and you're not Netflix"? The TL;DR; is those tech giants that have 1M+ concurrent users. They have a hard requirement for no downtime."

Actually, one of the more interesting parts of the Google SRE book was that they don't try to aim for 0 downtime. They consider the background error rate of any network request and optimising much beyond this is counter productive.

Even for individual services they make a point of not trying to make them perfectly available, as this means downstream services are less likely to build in adequate provision for failure.