How is a different (and longer) keyword "far more readable"? That's just a matter of preference and familiarity. The reason for choosing a different keyword is that it's not quite equivalent to an unless as the {} block must exit the surrounding scope. You read it like an assert statement with a custom handler.
> Moreover, there are many languages where an assignment or an initialization can appear in any place where an expression can appear. Such general rules are always better than special rules like allowing new bindings in a "guard", but not in an "if".
You can introduce bindings in an if too. The special thing about guard is that you can introduce a binding which is valid for the remainder of the scope outside the {} block (where the condition is true) but not inside (where the condition is false).