Too Long; Didn't Read
There are some services that need significantly higher availability than others. As a consumer-grade service provider, a 99.99% availability is likely to be sufficient. Above this, the consumers’ dependencies (of which you have no control) are collectively less reliable. This means that investment to significantly improve availability beyond this point isn't particularly valuable. Multi-cloud is a shorthand for building a platform that runs on both multiple cloud providers (e.g. AWS, GCP, Azure, etc.) This is the only way to be resilient to a full cloud provider outage - you need a whole second cloud provider.