The benefit to retaining the "edit comment" button after 5 minutes is that you have the opportunity to copy formatting. You can thus manually append to your old comment, which is a good thing. But this normally means deleting the existing comment and creating a new one.
This isn't necessarily a problem, but is unnecessary work. Appending to more-than-5-minutes-old comments should be allowed if there are no more-recent comments by others. If there are more-recent comments, the comments probably follow a conversation, and you should just create a new comment.
"tl;dr": If your intention is to reuse or amend content, there shouldn't be a need to edit, copy, and delete a previous comment, then paste and append to a new one.