The relevant time is right now. If the U.S. government had said "yes, we can do it!" and invested $20 billion in building an atom bomb in 1915, they would've pissed that money away. It would've been mindless optimism. Doing it after fission had been discovered was a calculated investment.
The issue is that we're quite a few "Nobel-worthy" inventions away from curing Cancer.
I mean, biologically we're not in the equivalence of 1950's physics. We're holding in 1300's physics.
I saw that someone once compared curing Cancer to landing on a moon without knowing basic Newtonian physics.
I expect that before we cure cancer, we'd have found ways to cure all "normal" diseases, have "designer babies" without side-effects, have "instant, proof-perfect medicines", etc.
I'd want his to work on those issues.
Right now it'll probably end up like Google X. A great lab, makes great prototypes, but practically get left behind by incremental technology.
> By the time someone wrote a check, the basic mechanism of the runaway fission reaction was understood.
I find that to be a gross simplification. I would challenge that the basic mechanisms of the fission reaction were as well understood then as we understand the basic mechanisms behind cancerous mutations now.
> There is no ready-to-go idea you can point to and say "if we spent $20 billion building this it's have a coin-toss's chance of curing cancer."
Over what time-horizon? We've achieved a great deal in the last 100 years that would have received less than a coin-toss probability if one were to be having this same argument in the early 1900s. The Haber Process [0] is a great example of something that we had no way of expecting we could accomplish (the energy efficiency aspect) before it happened.
What would you prefer Zuckerberg does with his money? I mean, he could build a massive yacht like Jobs before he died... As a species we have consistently accomplished feats that were as unlikely as the mitochondrial symbiosis that allowed us to exist in the first place, is there a reason to discount our abilities to continue this trend?
[0] https://en.wikipedia.org/wiki/Haber_process