This is already possible.
If you want a moderator to edit a comment, you'll want to explain that in a "Something else" comment flag. It's unlikely that a moderator will figure out that you actually only want half the comment deleted from a "No longer needed" flag—comment flags on sites that get any significant number of comments tend to be handled pretty quickly, and moderators rarely edit comments.
You can ask on the site-specific Meta site if the moderators there have a particular preference. Or you can flag some and see how they're handled—worst case, the flag gets declined, and it's probably not the silliest flag the mods have gotten that week.
Also, note that the remainder of the comment should be valuable enough to warrant taking the time to manual edit it with the necessary caution to not put words in the author's mouth. If it's not really adding much, it might be better off removed entirely.
Even worse is when a string of obsolete comments is concluded by a half-and-half comment. Deleting the first bunch would render part of the half-and-half comment obscured.
For cases where a large number of comments need to be deleted, you may also wish to flag the post and explain which comments should be deleted, as it will also allow you a longer maximum flag length. Different moderators have different preferences about whether they prefer individual comments to be flagged or the whole list—you can ask on the site-specific Meta about that as well.
Giving moderators the power to arbitrarily edit other user's comments is too much. But they should have the ability to remove a sentence or two from a flagged comment.
It's impractical to do one without the other, as minor phrasing tweaks are often needed when removing content (e.g., removing "Also, " from the following sentence).
Also, we can do things like fix dead links, typos, etc. Also, we've been able to do this for years and I can't think of an instance of anyone complaining about an edit to a comment (usually people are complaining about comments being deleted). Moderators have a lot of powerful abilities—as with the rest of them, moderators strive to use them in a way that earns the community's trust (e.g., not putting words in people's mouths).