Currently, decorating event listeners (kernel.event_listener) makes the decorated service private:
https://github.com/symfony/dependency-injection/blob/5d4f4ff02a3a5b24a7c81bb7e22f2e57c7526ee2/Compiler/DecoratorServicePass.php#L47-L58
resulting in an exception being thrown here:

This makes it impossible to decorate event listeners. (Also, I can't find any tests of decorating event listeners. Are there any?)