The language spec is the only place able to introduce meaning to a source byte sequence starting with a backslash that has been invalid before (e.g. not double-

The language spec is the only place able to introduce meaning to a source byte sequence starting with a backslash that has been invalid before (e.g. not double-backslash, or backslash-u). ${ on the other hand never been an invalid byte sequence inside a string literal, and was hence already used by many APIs. It's nothing but courteous by the language people to avoid avoidable collisions. Keep in mind that this new syntax is completely open to everybody else who might have a use case for strings-with-expressions, it's not like they are appropriating a previously blank spot for exclusive use.