Deleting segments (and categories) will not prevent you from viewing sent campaign reports. In the event that you delete a segment, you are deleting the grouping of contacts, you are not deleting contact records or campaigns sent to the deleted segment.
However, if you delete a segment the name of that segment can no longer be referenced in reports. For example, in the Tracking Statistics report (seen below) there will be a blank in the Segment column when the associated segment (or category) has been deleted.
If the segment identification is important to you, alternatives could include 1) naming your campaigns in a way that indicates the segment or category or 2) creating an archive folder in your segment or categories dashboards and move unused groups to that folder to remove them from view.
You can rename sent campaigns, if that is the route you wish to take. One way to rename a sent campaign is from the Tracking Statistics report. Click on the ID (far left column) for the campaign. Edit the "Mailing Name" field on the next screen (seen below) and click "Save".