Sarcasm aside, what's Starfleet going to do with all those 'gunships' built to fight a finished enemy?
already showed the Defiant
getting refitted for science missions. There are options.
And of course, just because the Borg are gone doesn't mean the UFP can beat all its swords into plowshares. While a return to a primarily scientific, diplomatic, and support role for Starfleet is much to be hoped for, it doesn't hurt to keep some powerful ships around in reserve, just in case.