There’s a complexity cost, though, right? It’s not free to run two different systems, manage two different billing systems, different tools, pay for cross-provider bandwidth, etc.?

I’m sure there are some cases where the cost is worth the complexity, but I don’t think it’s cut and dried as written here. Most cloud providers are very reliable, and my guess is that you are more likely to have a self-inflicted outage due to the complexity of your infrastructure than the cloud provider having an outage.