> [...] services run isolated as their own users [...] provides for a clear and simple mental model [...]

How is that a clear and simple model? Are email or printing users?

I think the whole discussion is futile without having a common understanding of what we are talking about. That is:

- What is a user?

- What is a group?

- What is a role?

- What is an account?

- What is a service?

- What is a job?

- What is a process?

- What is a container?

- What is a namespace?

Moreover, you cannot say whether an abstraction is good or bad without knowing what our goals, use cases or target users are.