> It seems to me that people tend to vastly overestimate their uptime requirements. "Real time 'web application'" used by hundreds of millions of people can be down for hours and yet succeed wildly

That may be true for social media apps where the Terms of Service don't include any SLAs/SLOs, but if you're a SaaS company of any kind, the agreements with clients often include uptime requirements. Their engineers will often consider some form of "x number of nines" industry standard.