I agree with you, but the big difference between function arguments and effect systems is that the tools we have for composing functions with arguments are a lot simpler to deal with than the tools we have for composing effects.

You could imagine a programming language that expressed “comptime” as a function argument of a type that is only constructible at compile-time. And one for runtime as well, and then functions that can do both can take the sum type “comptime | runtime”.