[cowsandmilk]

> which means you're paying 3x more than you need to 99.99999% of the time.

This would be a poor decision. If you assume AWS, GCP, and Azure would fail independently, you can pay 1.5x. Each of the 3 services would be scaled to take 50% of your traffic. If any one fails, you would then still be able to handle 100%. This is a common way to structure applications. Assuming independence means that more replicas result in less overprovisioning. 1 replica means needing to provision 2x. Having 5 independent replicas means, you need to provision 1.25x to be resilient against one failure as each replica will be scaled at 25%.

In general, N replicas need N/(N-1) over provisioning to be resilient against one replica failing.