I did the math for our own stack and after a setback month in client revenue, and decided to put all our servers into a single AZ in a single region. The only multi-AZ, multi-region services are our backups. Surviving bad machines happens often enough that it's priced in via using Kubernetes, but losing a whole AZ is a freak accident that's just SO rare that calculating real business risk, it seemed apt to pretend it just doesn't happen (sorry, Google Cloud Paris customers).
Call me reckless, but I haven't looked back ever since and it saves us thousands of dollars in intra-AZ fees per month alone.