> I think what a lot of people fail to understand is that there are certain categories of apps that simply “can never go down”

I refuse to believe that this category still exists, when I need to keep my county's alternate number for 911 in my address book, because CenturyLink had a 6 hour outage in 2014 and a two day outage in 2018. If the phone company can't manage to keep 911 running anymore, I'd be very surprised what does have zero downtime over a ten year period.

Personally, nine nines is too hard, so I shoot for eight eights.