Because the reasons which many fans wish to see the demise of Rangers aren't to do with football. It's what Rangers represent for many in Scotland, that they want them to end.Agree with you. Both Celtic and Rangers need each other if they want the league to keep growing. If Rangers were to somehow fall then it will have a huge negative impact on Celtic too. Dont seem to understand why Celtic fans are actually happy about the demise for both footballing and financial purposes
Good example will be Real Madrid and Barcelona. If one was to fail then the other would surely be overwhelming favourites to woin the league which will in-fact decrease the quality of the league etc.
So a lot of the arguments put forward here don't really apply to the situation.