From my quick glance, the reasons it can't be a macro are for better error messages and to prevent abuse? I feel I'm missing something because that seems somewhat weak of an argument.
I'm really liking the idea I saw on reddit for UFCS for macros. Instead of creating a special way to declare postfix macros, allow any macro to be postfix.
let bar = "Hello {}".format!("world");
let bar = foo().async!()?;
let bar = foo().yield!()?;
let bar = foo().r#try!();
let bar = foo().dbg!()?; // EDIT Someone else posted this other good example
I assume it isn't an edition-level breaking change to take a keyword and unreserve it if we decide macros can work and postfix macros solve the problems.