When Ray Franz left the GB (still an active JW), things were different then. He had a friend who was able to give him a place to live and employment to support himself. The disassociated status of his friend was a non issue.. at first, anyway.
Any GB member today has presumably spent most of his adult life at Bethel. It's unlikely he has any wordly contacts so if he were to defect he would have no safety net. No JW would hire him or give him a place to live.
Any GB member even rolling the idea around in his head would have to be assured in advance that someone would be there to catch him, offer him a room, job, something... anything.