I absolutely have a UPS in my basement. Considering how little runs on it, it's pretty cheap to get a long runtime out of it too.
A lot of things I see with millions of concurrent users aren't actually monoliths: Sure, Facebook needs to handle that. But most cloud apps would better be run where each tenant/business/team operates an Intel NUC in their basement, instead of the developer using the cloud as a way to force rent-seeking behavior.
Unless your fiber has an outage, then you want redundancy, multiple independent uplinks that is.
And if your whole region has a problem, which is more likley to happen than one might think, then you want a multi-region setup e.g. us-west-1 and us-east-2, and then we can start to calculate the numbers of nines, unless your username is ocdtrekkie, he can beat AWS with a single NUC while he is sleeping.
Including Google. Who have now hit a point where that's no longer sustainable, and developed a set of best practices to ensure reliability beyond what you can expect from a NUC in the basement.
A lot of things I see with millions of concurrent users aren't actually monoliths: Sure, Facebook needs to handle that. But most cloud apps would better be run where each tenant/business/team operates an Intel NUC in their basement, instead of the developer using the cloud as a way to force rent-seeking behavior.