I see daniel-p's point about just how many folks are effected by the blood ban, but I would argue that the civil service issue affected as many. I know lots of folks in my parent generation that went to jail in the US over Vietnam when they could have been spared with performing civil service at home instead. Those boys who went to jail when they were 18 are still elders and such and didn't leave over the issue. I guess the main difference is the loss of life (obviously not even a real comparison).
I guess the closest comparison would be the allowance of fractions now vs the previous complete ban. I don't know a single person that left because the WTS allows "fractions". As a matter of fact, when the big change was made, there was a look of relief. Think of all the folks that could have been saved in the past if the now allowed fractions were allowed then! I know that much of the fractioning science is relatively new so maybe that portion of the ban didn't matter as much.
Let me ask, how many lawsuits have there been? I know the Hughes suit but Canada's courts seemed to have come down on the WTS side. You know the US courts will favor a religious body over an individual due to the 1st amendment. And since the US is by far and away the most litigious country in the world, I don't think the WTS is worried about losing too much.
I do believe that the perception of major liability is very concerning to the WTS. I think that countries in the EU, Russia and other places view the WTS with suspicion and have honed in on the blood ban as a reckless endangerment (which I agree). The old version of the WTS would try to get themselves thrown into jail. This version doesn't have any fight in them and if there's a change in policy, its probably going to be to try and be seen as a legitimate religion.
I agree with metatron in that most of the rank & file will say that "blood is still bad but the WTS can come out and say it due to Satan's wicked legal system". When I was a congo secretary, I was shocked at the number of folks that refused ALL fractions on their blood cards. I saw probably half of the congregation's because JWs will often provide a copy for the congregation's files in case of emergency. I also saw a lot of folks VERY confused as to what to do. I always recommended praying (of course) about it but when asked what I did, I gladly told them I said yes to everything. Immediately, folks that I had that conversation with decided if an elder said yes, then they would too.
Bottom line in my viewpoint the best that can be hoped for is making RBC, WBC & plasma fractions and allowed and keep the whole blood ban for another 5 to 10 years and then finally dropping. But, there will still be a large number that will continue to refuse those.
I really don't think they would lose membership over the issue. I think there will be a shrug of the shoulders and folks will continue on.