users want to do things, if their goal depends on a complex chain of functions (provided by various semi-independent services) then the ideal setup would be to have redundant providers and users could simply "load balance" between them and that separate high-level providers' uptime state is clustered (meaning that when Google is unavailable Bing is up, and when Random Site A, goes down their payment provider goes down too, etc..)
So ideally sites would somehow sort themselves nearly to separate availability groups.
Otherwise simply having a lot of uncorrelated downtimes doesn't help (if we count the sum of downtime experienced by people). Though again it gets complicated by the downtime percentage, because likely there's a phase shift between the states when user can mostly complete their goals and when they cannot because too many cascading failures.