For prose it’s usually desirable to allow source text to contain newlines while the resulting rendered content fluidly adpats to its container or whatever styles are applied to it. This is particularly important once you consider the growing diversity of screen sizes used to consume content.
We can’t compare writing in a markup language with writing in a WYSIWYG environment. Of course in Word pressing the Enter key results in a new line, but when we’re manipulating the source “code” for text, WYSIWYG features like this would muddle the waters and make the outcome less predictable overall.
For that reason, IMO not even the line breaks should be considered for a (default) feature – but perhaps we can envision a mode that can be activated within a block (like HTML’s <pre>formatted text). Even then, line breaks using backslashes are so easy to produce that I don’t think such a block would be warranted (in the base spec, that is).