I think at least 5% would disassociate. I don't think it would be as much as 50%. But I believe that about 50% would probably immediately stop shunning disassociated relatives, and that number would grow gradually until shunning of disassociated relatives ceases altogether or becomes just the aberrant behavior of an over-zealous minority.
The thing to keep in mind is that there are other repercussions to disassociating, apart from shunning, and heavily indoctrinated Watchtower loyalists will impose these threats even without being prompted to do so by Watchtower. Some are employed with other JWs and risk losing their job. Some would still risk being disinherited. Some would risk being kicked out of the house. Some would risk being divorced on the trumped up grounds of "spiritual endangerment".
For this to really do good, Watchtower would not only have to remove the shunning sanction, but actively teach JWs that it is wrong to shun disassociated ones or punish them in any way. Here's a scary thing to think about: Faced with legal pressure, Watchtower can officially put an end to their shunning policy tomorrow without there being any relief from shunning for X-JWs. They would just have to print something like this:
"In the past God's people have been given repeated Bible-based counsel on the need to "quit mixing in company" with those who are unrepentant sinners, or who renounce their Christian dedication by written word or action. (scripture citations) However, today, more and more governments are concerned with enforcing human rights and rights to freedom of religion. Some interpret our faithful adherence to the scriptures' injunction on not associating with disfellowshipped and disassociated ones as being unduly strict to the point of violating human rights and freedom of religion. Hate laws once used to prevent the verbal incitement of hatred toward minority groups are now being used to restrict persons and organizations from publishing religious direction that the state deems as hateful and/or intolerant. ("framing trouble by decree" scripture citation) How should God's people respond to such developments? Jesus taught his followers to be "innocent as doves but cautious as serpents" (scripture citation). To this end, our publications will no longer feature the direction to quit associating with disfellowhipped or disassociated ones. We no longer give the direction to cease communication with such ones. Of course, true Christians know the Bible's direction on such matters and endeavor to follow their Bible-based conscience regardless of the direction or lack of direction given by any individual, government or organization. ("we must obey god as ruler rather than men" scripture citation)"