alt Hacker NewsAWS had switched from using something like this ("injection tokens") to just regular IAM roles, though managed by the AWS.
The only special permission that services (actually, the AWS accounts that they use) inside the AWS have is access to "service principals". The service roles inside customer accounts then use them to grant access.
AWS IAM is painful, but it shows that you can design a secure permission system.
You can add many layers of indirection, but unless you're actually authenticating that a system service is using the credentials (and not, say, a user or a script) then it boils down to a long-lived token at the end.