My argument isn't that Celtic need a Rangers* to survive, they don't, but they do need one to thrive.
Your Chairman/BoD's aren't going to sanction £9m purchases of the like of Eduard or £7m on Julien simply because they won't need to in order to beat the likes of Aberdeen, Hearts or Hibs. They won't take this gamble on the hope of making the UCL group stages either. This in turn will be seen negatively by the supporters, many of whom will lose interest (as seen 2012-16) and attendances will fall off.