From an anthropological viewpoint I would say that the sole validation of a paranoid belief which is held collectively, (as with JW org) is the number of believers. Enough believers gives credibility to both insiders and outsiders. To insiders it is a confirmation of divine attention, and large numbers give the organisation a superficial respectability to the general public.
I think that the reversal of numbers will signal to the remaining ones on the inside that the magic has gone from the religion, this notion could be contagious! If enough sheep leave many more will follow them, the JW religion after all is just about appearances; witnesses only do what they see their brothers and sisters doing. Starting as a trickle departing numbers could become a cascade eventually leaving only the most unquestioning devotees back in the forlorn KHs. Enough leavers would spell the end for JWs-- hence recent and desperate emphasis at assemblies on being loyal and staying in the org.
Of course The gb are all about control, they tell what to talk about and what to believe in through their propaganda on TV and in the Watchtower. It is said that the worst enemy of a cult is heresy, just look at Paul's hateful rantings against people who disagreed with him and his rules in the first century and this is because Christianity was always a superstitious and contentious cult until Rome created its own version and gave it sanction of orthodoxy.
So the JW org praises its slaves and hates the leavers due to the visible evidence of their symbolic declaration that the Watchtower is wrong. Numbers of believers are the key factor in cult strength.