It's not just slightly more expensive. And you have to consider substitution effect. If you take the more expensive route and it takes 10 years longer to deploy, then there will have been another 400K car collision deaths in just the US, and over 10 million in the world in those 10 years that could have potentially been saved. So was the delay for the safer product worth it? The only reasonable answer to this question is "I don't know" because you can't predict how much safer the expensive system will be and how much longer it will take.

The more important question is how many people are killed by non-autonomous cars in the same situation. It is inevitable that someone will be killed by a self driving car sometime - but we already know lots of people are killed by cars. If you kill less people getting autonomous rolled out fast than human drivers would that is good, but if you are killing more people in the short term that is bad (even if you eventually get better)