That really sucks man. Whats the point if your character just gets erased...
The game is not a sandbox. There isn't really anything left to do after you've beaten the final boss, so no real reason for things to continue. What would you do, go back to Arx which you've already cleared? They could have done that as an epilogue section (letting you talk to people), but it still wouldn't be a return to gameplay unless you simply hadn't finished most of the content beforehand.
The other option would be to have a postgame dungeon, but here really all that would be doing is extending the game for one extra act past the story.