My experience with very large scale B2B SaaS and PaaS has been that customers like to get money, if allowed by contract, by complaining about outages, but that overall, B2B SaaS is actually very forgiving.

Most B2B SaaS solutions have very long sales cycles and a high total cost to implement, so there is a lot of inertia to switching that “a few annoying hours of downtime a year” isn’t going to cover. Also, the metric that will drive churn isn’t actually zero downtime, it’s “nearest competitor’s downtime,” which is usually a very different number.