You are correct. At the moment, the best you can hope for end game is 'Outpost Master'
Which resets all the Outposts only. Alarms still work, so if you manipulate the NPC's enough, you can have a steady stream of enemies to try an complete challenges.
Otherwise, so far, there is built in way to repopulate the world. You're stuck.
Don't finish the game
Youtube the ending
Uninstall....