I agree with your sentiment, but we also need to careful with hindsight bias (i.e. it's easy to pick the winners after the fact)! It's a tricky question!
The upside is immense even considering just existing diseases, but we are really bad at pricing things that improve the status quo vs prevent bad things.
The theory, assuming a delivery mechanism is found, is rock solid---"if not when"---and I therefore wouldn't call it a gamble.
On the other hand, I don't know enough about the biology to speak to the difficulty and uncertainty around delivery mechanisms. Can we get more info on that? I suspect it wasn't too uncertain, though "oh, we change this base pair a bit and the immune system doesn't care" does seem like a relatively unplanned discovery.