The JW parent organization has made many adjustments over the years to blunt the sheer human cost of adherence to this doctrine. Nobody thinks twice about accepting gamma globulin today. It's the basis of most post-exposure vaccines. It's the basis for antivenins for snake and other poisonous bites. It's used to treat acyte thrombocytopenic purpura, Kawasaki's syndrome, etc. Yet there was a time when JW's were not supposed to accept gamma globulin.
This is why anything that's based on a simple calculation is fundamentally flawed.
A true model is needed taking into account the different factors of both WatchTower doctrine AND medical treatments available as well as the number of JWs around and the likelihood that they would refuse blood treatment and die as a result of it as each period of time (whether per year or per decade).
I also don't think it's unreasonable to suggest that years ago, when the JWs numbered less than 1 million, they were more 'die hard' than now. I could see a larger percentage refusing blood in years gone by than now.
Heck, I don't think you'd even get many JWs willing to sit through 7-day, 8am-9pm assemblies anymore like they used to!
Simple logic: if the period the study covered was the worst period for JWs dying then you can't apply the results to ALL periods. Does anyone truly believe that the risk has been constant and that changes in doctrine and treatment have no effect?