Not a security expert here...

But Discretionary Access Controls is a standard part of OS design for a very long time.

It is certainly possible to go back to DOS-days and run all your programs without controls as terminate and stay resident programs. But that would be awfully inconvenient.

The concept of "users" isn't just for human users. It is used to do things like prevent your web server from being able to read and edit your password files and such things.