Would you rather that one person's lifespan was reduced by 20 years...

You can convince someone that there life will end or be ended.

And this will raise emotional and moral considerations.

But their lifespan will still only be the duration of their life from conception to death.

I did not mean to impose you to share your perspective from a moral view, I was only curious in your perspective if you were willing. On the other hand, many of the forums in philosophy are theoretical because they are only philosophical and not be taken as solid evidence but rather intuitive insights. I recognize you were taking a practical approach to the question and exposing how life could never be shortened (I agree), but I believe this was intended to only be theoretical, so the practicality of the idea is irrelevant. By all means, please share your practical perspective and insights, I'm merely saying the practicality does not make the theoretical question irrelevant, but impossible to implement. Regardless, it can be viewed as a mentally stimulating exercise which could include a moral, unless the topic author requests a particular approach to the question it is interpretable.

OK.

But if the proposition is fundamentally flawed.

I reject the question and all narratives derived from it. 

Even if the hypothetical situation is not feasible, it can still offer valuable insights. For instance, exploring whether life is equally significant regardless of whose life is shortened, or considering the opposite perspective, may provide us with a unique perspective towards morality. Our aim is not to solve a specific problem for future use, but to engage with this intellectually stimulating concept to gain novel perspectives and enhance our comprehension of the world.

Why do you reject the question and any narratives derived from it? Is there a specific reason that the question makes you emotionally withdrawal, or perhaps have you already answered this question before, and you disliked the insights it provides? You seem to be taken back by the question and unwilling to understand what it presents.