Without disagreeing with your point, there are actually currently gene therapies to cure trisomy 21 [1] (a clever trick of affixing the signal that is used to silence a (female's)/the (male's) X chromosome to the 3rd of the 21st chromosomes). I don't think it's all the way through FDA approval, and is still very much experimental, but is exactly the kind of thing that is curable. And that is where your point stands: it is hard to know which unknowns are easy unknowns, and which unknowns are nearly impossible unknowns because both are currently of the same quality - unknown. The grandiose claim is fancy, and much will be accomplished in 80 years, but even ill-defined language of 'disease' and 'cure' belies our ignorance.
[1] http://www.biospace.com/News/down-syndromes-extra-chromosome...