Just like you setup multiple instances across different availability zones for redundancy, you also need multiple billing contacts in case one of them fails and causes an outage.
Not sure how to mitigate these risks for providers that only support one "project owner" with a single billing address :/
But that's the same if you have multiple billing contacts. What is required is that the responsibility in the group of people that receive those notifications is clear.
Having multiple people that receive billing notifications is strictly better than having a single person - what happens if that person is on vacation, sick, overloaded? No one even has a chance of catching this. If at least one other person receives those increasingly important emails, they at least have a chance to catch this before it breaks.
Have one person clearly in charge, but as the emails get more frequent with increasingly dramatic titles the other people can check in and ask if everything is under control
Not sure how to mitigate these risks for providers that only support one "project owner" with a single billing address :/