People whine about how slow the pace of research is, but that's not because clinical trials are so slow - it's because most drugs fail. The vast majority of drugs that work in animal models get bounced in Phase 1 (when they show unacceptable toxicity in humans so great it can't be dosed) or Phase 2 (where it just shows no effectiveness in humans at all - not even some level of effectiveness that fails in cost/benefit analysis). this is the problem with people complaining "But every delay kills people!"
No. You don't know going in whether it's going to work - that's why you're doing a trial. And from a naive position, the probability is pretty clear that it's very unlikely it's going to work. There's absolutely no reason to think any particular drug is worthwhile, and every to worry about its side-effects. With a sample size of 12, this one may yet became a last-ditch drug with toxicities worse than chem/rad. We don't know yet. Using this is an example of over-cautiousness is ridiculous.
Additionally, the slowest part here wasn't the clinical science - it was the basic science. Gene therapy has been a failure for decades, and we've been repeatedly going at it to try and get it to work. It's not as though this is the first clinical trial for a gene therapy: it's just one of the first to be worth anything.