As someone who has come back to the game after a year's hiatus and has since been playing daily, I would not like to come back to see my airline forcibly rebuilt/bankrupted.
Maybe a better way would be to not include their routes in pax sim calculations, pause their airline state, after a certain period of inactivity (e.g. 3 months is fair to me), and unfreeze their routes once they come back. That way, they won't lose their bases, their routes, their money, and it wouldn't cause a strain on the simulator, and virtually frees up routes for newer players.