This is one of those hypothetical questions with no real answer; the WTS is not going to make any changes this significant, so we'll never know.
However, since we're speculating.... I think the shrinkage would be far less than you'd imagine. Why? Because the very changes that eliminated any penalties for leaving would so change the organization that it would enhance the JW experience rather than detract from it.
To change the structure so that there were no penalties for viewing the "truth" and its doctrines as "optional" (see "Catholic Church") would make associating with the dubs more palatable, not less. What would shrink would be f.s. hours and meeting attendance. But they'd still be able to draw a crowd. The changes it would take to allow "association" to be a conscience matter would change the very structure of dubdom. Many who long to shed the high control tactics, the cronyism, the lack of love, and the endless treadmill of meetings, service, study, and all the rest, would have less incentive to pack up and go - once those characteristics of the organization disappeared.
The organization would be a far better place with just a few major changes. Make meeting attendance, field service, medical treatment, and abhorrence of holidays truly conscience matters, and return the selection of elders to the local congregation, and dubs' esprit de corps would go off the charts. I imagine many folks on this forum who've left JWs who would go back if radical changes took place and the dubs became a live-and-let-live religion with mainstream characteristics and values. After all, most of us left family and "friends" when we broke our JW ties.
But, of course, this isn't going to happen. They'd die rather than change.