I wanted to correct one letter in a name. This was impossible because there was only this one typo. So the typo persists. Shouldn't this rule be dropped?
This rule is present on all Stack Exchange sites for suggested edits. The reason is that very small edits like this still requires multiple users to look at your edit and approve it. For very minor edits like this, it typically is not worth the reviewers' time to do so. Users with the edit privilege (currently 1K rep) can make minor edits like this. For more information see this post on meta SE. If you really want to propose a change to this you should do it over on meta SE as we don't have any control over it (but don't be surprised if it is marked as a duplicate or rejected quickly).
You should try to find other substantive improvements to the post which would raise the edit above that limit, or leave a comment mentioning the error. If there really are no other issues, there are hackish ways to add characters which don't display anything, like
(123456) but I recommend avoiding these if at all possible. Personally I'd be very likely to reject an edit like that unless it is crucial and obviously correct.