Here's one epidemiological study:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1734744/. Median life span was 18+ or 34+ years depending on the exact nature of the genetic deletion. That's without taking into account ongoing improvements in care, or that Amelia has already survived the high-mortality infancy period.
One of the doctor's state concerns (according to the Riveras) was how Amelia would manage the post-operative regimen 30 years from now. That doesn't sound like he was concerned about her having a too
short life expectancy, unless he was throwing every possible objection against the wall to find one that would be convincing.
Parents not hearing something is a possibility I've acknowledged, but I have not heard any plausible suggestion for what it is they could have missed, or why if they missed something medically decisive the doctor chose to emphasize "because quality of life". A poorly informed MD (poorly informed about developmental disability and how it's cared for) is basically my own hypothesis.
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One more thing: I should mention that had the Riveras decided not to have the operation, I would be the last to object to that decision. (Of course, in that case I never would have heard about it, but in principle.) I am not a believer in life-at-all-cost and I have NO rosy illusions about what the Riveras are in for no matter what transpires.
From
http://www.ncbi.nlm.nih.gov/books/NBK1183/:
It's hard to imagine what that can do to a family, but I don't have to. I've seen it many times over and lived it personally.
But it's their decision.
Respectfully,
Myriad