There are various levels of outages you may encounter with an etcd outage, but it shouldn't take your workloads completely offline. The cluster API will be offline, but the workloads will keep on chugging. I think with the decoupling of etcd and the expectation that this might happen, we'd see more improvements on how to (gracefully) handle these situations.

Also, there was a bug with kube api where it wouldn't failover to other etcd members during an outage. So I would say most customers have ran kubernetes with a "single backend" at some point.