After all a DF or DA person is considered living under a death sentence in the JW world........
I think that was is an old belief as is "spiritual leprosy" used to descibe DF. The JW refer to DF as a condition. But more accurately an unrepentant sinner because that is the only reason why anyone should be df. THe DF form s77 alludes to 777 or forgiving 77x 7.
From an objective position though, there should be some way to deal with lawbreakers in any society even in JW. DF like jail or punishment or a fine is an attempt to force a lawbreaker to comply and is it serves as a warning to deter others, otherwise you would foolishly have everyone doing whatever they wanted hurting themselves an others( based upon the related rules of conduct) without any fear of punishment.