They may have an initial drop, but I think they would recover and maybe grow in the long run (especially if they drop the blood doctrine as well).
They're not in it for the numbers. They do it for the power.
Power over the lives of people. That is a massively powerful drug. Dictators go to war and kill thousands or millions of people because they must have that drug. The GB are addicted to it every bit as much as is Kim Jong-un.
Of course, the more people you have power over, the better, but for people of this ilk, better to have strong control over the lives of 8 million than weaker control over 20 or 30 million.
I think the GB would prefer to have even stronger control, even if it meant losing a few million current JWs. I expect their policies and procedures to get even nuttier as time goes on...if they lose a few million fence-sitters, but the remaining 4 or 5 million are even more insanely loyal than they are now, I think they'd be perfectly happy.