To be honest, I don't know that we need this, except to keep comments from going over the limit (and not many people are going to be in favor of that, given comments' second-class status, etc).
After all, the full link works as a URL without a page reload in my browser, and is easier to construct (simply right-click the timestamp of the comment and copy URL) than getting just the # part of the URL (which involves an additional step).
Now, and again in my browser, the title changing does not break the link (example - I put two linked comments there, one with the old slug, one with the new slug). Both links continue to work, within the page or from anywhere, even though one contains an outdated slug. When the title has changed, it does force a reload on the same page, but this seems like an edge case I'm willing to live with - again, comments are not meant to be permanent, and I wonder how often we need a link to a comment in the same thread to be maintained, especially when most of those comments are probably obsolete once the question has been edited.
In the end: they're comments. With links to other comments. On the same page. Neither of which should last forever. If either comment contains useful information, edit it into the post.
Can you please cite multiple examples where this functionality is demonstrably useful?