I have done extensive work with monoliths, and extensive work with microservices. I quite literally have never seen microservices be a benefit in the way you are describing, and if anything microservices can make the problem of robustness more difficult, because in many workflows there are one (or many) services that are critical. If they're down, everything is down, and if anything distributing these dependencies across different systems can make it more difficult to determine where the critical points of failure are.

As you point out, there is nothing inherent in a monolith that makes it less robust. I currently am building a monolith that is deployed on auto-scaling servers on a cloud provider, so if one server dies it just spins up other instances.