The problem with this is that we are all dying, from the moment we are conceived. It is just a matter of when.
I think that Pope Pius XII’s ALLOCUTION TO MIDWIFES covers this very well, but I’ll try another analogy.
Why does your seem reasoning not apply to euthanasia of the critically ill elderly for financial need? They are dying, and quite likely don’t want their lingering death to drive their familes to financial ruin…
Our core belief is that we are each a unique creation by God, who loves us each infinitely. We are also prohibited from judging relative moral worth and warned that our idea of living in grace and God’s are likely different. When we make ‘pragmatic’ decisions to sacrifice the weak for the strong, or other decisions on who should live and die, we defy these teachings. We also embrace evil means, which the Church warns us, makes us evil in turn.
The Church has also argued that this thinking is a slippery slope to other grave evils, like eugenics. We tend to forget that the eugenics movement thrived in the US and was even supported by the Supreme Court in a contrived case. It took the holocaust to make many people realize that they were pursuing mortal sin, and even then, forced sterilizations occured in some states until the 1970’s, when the Supreme Court again looked at constitutionality.