Disfellowshipping is arguably WT's most powerful control measure. Allowing for even "discreet" association with a DF person undermines the fear that keeps the R&F in line.
Aside from that, think about WT's standard argument for why the DF arrangement exists:
1. It's a "loving arrangement" because it shakes the DF one to their spiritual senses, and
2. it protects the congregation, the family members, and proves our "obedience" to Jehovah.
If WT now allowed associating with a DF one to be done "discreetly", it opens a big can of worms:
1. Does the WT not "love" DF people anymore? After all, shunning them was proof of WT's "love", so allowing association must mean they no longer love those people.
2. Does WT no longer care about "protectiing" the flock? After all, the across-the-board mandate of shunning was for the flock's protection. If WT allows association, it must mean they no longer care about protecting the flock.
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Besides, doesn't "discretion" really mean do it on the sly, pulling a fast one, being 'secretive', putting up a false appearance? WT has historically condemned this type of behavior, classing it in with lies, deception and leading a "double life".
Finally, if associating with a DF person "discreetly" is okay with the Great Hypocrite, Jehovahâ„¢, why didn't he just tell the WT this from the very beginning and save everyone decades of grief?