> but the interpolated version seems clearly simpler to me. What's the disconnect here?
The disconnect is this. An explicit list of variables to use for interpolation is not redundancy. Because the placeholders does not, by themselves, refer to anything. Placeholders are just holes and only have a meaning in the context of formatting functions. And even in them, they does not refer to a local variables, even when they share a common name. So when you use .format() function, you are actually saying, "here is a string with some named holes. Fill the hole named 'A' with the value from variable 'A', the hole named 'B' with the value from variable 'B'.
So hole 'A' and variable 'A' are different things even if they share the same name.
Now, the simplicity argument.
Every thing should be made as simple as possible. But not simpler. Why not? Because when you simplify further, you are paying a cost (often not apparent initially), some times in clarity, sometimes in correctness and so on. The pigeonhole principle.
In our case, we can further simplify the process by adding an implicit mapping from named placeholders to local variables. When you do so, you are adding something implicit. The costs of which might not be apparent at this point..
So this pep is strapping on something to the whole language, to slightly simplify this one use case. Which is why I said that it is trying to simplify it further than it is possible.