Suicide and Refusal of Life-Extension

What is the difference between the refusal of life-extension procedures and suicide? Suicide is the act of intentionally causing one's own death. By this definition, the two seem distinct. But is there a philosophical difference between an act that reduces one's potential life span from X to zero, and refusing an act that increase one's potential life span by X?

The idea seems to be that there is some natural life-span whereby it is immoral to willfully reduce, but perfectly acceptable to refuse to extend. This seems indefensible. If suicide is deemed wrong because it robs us of future choices (as per some arguments from liberty), then it is also wrong to refuse life-extension as it prevents us from enjoying the same future choices. If life is sacred (as per deontological argumenst) and has value in itself, then the loss of this life due to the non-adoption of life-extension procedures is also immoral.

Now, consider this thought experiment. Assume that one's remaining life span can be perfectly predicted in advance, and that that amounts to X years. Now, also assume that there exists some full treatment that can increase one's lifespan by Y years. One also has the option of taking a partial treatment that has a lesser effect, and extends life by only Z years, where Z is less than Y. Now, there are four people.

Person A refuses treatment, and lives X years.

Person B accepts partial treatment, and lives X+Z years.

Person C wants a partial treatment, but the center only offers full treatment. He accepts full treatment, but at the same time he swallows a poison that he knows will kill him in X+Z years time. He lives X+Z years.

Person D accepts full treatment. After X+Z years, he kills himself. He lives X+Z years.

If we accept that A is not immoral by his refusal of the life-extending procedure, then we must accept that B is not immoral by choosing a partial treatment. There is little practical difference between B and C; both live an additional Z years. Can we consider C to have committed suicide? While he has intentionally caused his own delayed death, his motivation is not to die, but instead to extend his life by Z rather than Y years. Therefore we do not consider it suicide.

What about D? Most would class it under suicide. But what are the philosophical differences between D and B,C? In all cases they do not want to live past X+Z years; the difference is only that D has made his choice rather late.