Even perfectly designed systems can have flaws. In your design you assume frontend part will have 100% uptime - practice shows that your hosting just can't provide it. AWS, GCP, Azure, you name it - all of them have failures.
Your statement holds true if you use a single cloud provider. FTR we ran our own DCs with several AZs spread globally. We did suffer failures in individual DCs occasionally but there was zero user-visible impact which is the whole point.
Sometimes mistake in router/balancer rules can make your servers non-accesible for users. Really huge systems sometimes can't be federated. I agree we need to design systems to be fault-tolerant and high-available but I also know there is no recipe suitable for every system.
Agreed. What I said does not protect against BGP blackholing or ISP screw ups rendering services inaccessible. However, all I was pointing out that services could be built for higher uptime than what is currently being offered.