Any service running way over quota could break all other services. (at the end of the day, resources are physically limited to what you have in your datacenters

Any service running way over quota could break all other services. (at the end of the day, resources are physically limited to what you have in your datacenters) Quotas are one way to isolate this impact to that service in particular. Of course, when it's a critical service like authentication, it hardly isolates anything... but I can't think of a better alternative.