You really haven't a clue have you?
No one is changing their mind for no reason - you get a dx, you decide to keep it, you only then change your mind if something else changes that affects that decision... Like, further information suggesting prognosis much more severe, like life circumstances changing that mean what you could have coped with, you now can't...
So your first scenario, it just isn't happening - people are not deciding to terminate late on a fucking whim. They cannot, you realise there needs to be the agreement of relevant medical people for this to happen, so there is no situation where someone up and says 'ah no, I fancy a termination' and everyone falls into line.
As for 'chance at life' - still no, then you're into quantifying how much life, quality of life... five minutes of agony? ten minutes? a month of being locked in, in pain, unable to communicate with anyone? a year? four years? forty years but only able to roll your eyes and grunt, and everyone you depend on runs about guessing at what you want, think, feel...
Unfortunately, the human body and medical science advances mean it is possible for someone to be alive, and yet have next to no ability to do anything at all, for a very long time - at obviously, enormous expense both financially and emotionally and in terms of hours of care.
So 'alive' and 'surviving' are not indicators in themselves of a life having quality and being worth living.