I somewhat agree with TS. There would not be an "immediate mass exodus". Most people who are still in are pretty well programmed to follow the same routine.
Just as an example, the WTS has been practically begging JWs (albeit subtly) to fill in their anti-blood DPAs to allow fractions - yet I'm sure well over 50% check the box saying "no fractions". The "no blood in any form" meme has them by the throat.
However, within a few months of a "no more DF" policy, a trickle would start just not showing up any more at meetings. Eventually the trickle would grow to a stream, as people would see there were no repercussions for just stopping.
Eventually, within probably 2-3 years, the JW population would probably half what it is now, or less, at least in the US & western Europe.
I think in 3rd world countries the positives of being a JW outweigh even the harsh negatives of DFing, so the exodus would be slower there.