Wanting to trade places with someone who truly wants to live is an understable phenomenon. It is incredibly tragic when someone who is happy, has zest for life, and is excited to keep moving forward is taken abruptly and against their will.
Correct me if I'm wrong, but I believe it's the definite knowledge that your time is approaching is what you seek, rather than the physical suffering entailed by terminal illness. Or perhaps, the support that is provided for people with fatal diagnoses which is sparsely given to those with more benign conditions? The finality resulting from such a prognosis is one of the most grim, soul crushing things I've ever had to witness.
My grandfather had a fairly good life, up until his long term health condition became terminal during my teenage years. The state he was in during his last month was so harrowing, that even walking into his room and laying eyes on how enfeedbled he was would nearly bring me to tears. Over time, he lost his ability to breathe properly, and the lack of oxygen would make him delirious and completely out of it. Once you hear the death rattle, it's almost permanently burned into your memory.
The last days of his life traumatized me immensely. I will never forget how decrepit he was at the very end, and how those waste of space hospital staff treated a dying elderly man like a burden they couldn't wait to be rid of. To them, the loss of life was garden variety and nonchalant, assumably because they'd witnessed the same scene play out countless times in the past. There was no dignity in that environment.
Euthanasia is banned where I grew up, and my grandfather would never have entertained the possibility due to devout religious beliefs, so an option like that was completely off the table. If you're lucky, a sympathetic doctor might slip your loved one a little too much morphine once they are in palliative care.
I don't think anyone would willingly wish to experience that hell. On the other side of the fence, I had another family member suffer for years with an autoimmune condition that doesn't severely decrease the lifespan of most people, only to succumb to the agony it caused after losing her entire youth to chronic, incurable disease.
Despite having organ failure, doctors would not accept her quality of life was nonexistent and continually forced endless treatments, psych hospitalizations, and surgeries upon her until she ultimately died in misery anyway.
Her demise was just as tragic and senseless as others I had seen, except there was a certain solemness in the fact that she hadn't gotten to experience the happiness of a life well lived that my grandfather had before he became ill. The majority of her life was spent suffering and fighting a battle that no one could win.
Both situations were tragic and heartbreaking in their own ways, irrespective to each other. There's no use in comparing a subjective experience like suffering or trying to weigh it on any sort of objective scale. I don't think anyone can truly understand one or the other unless they have first hand expertise, and even then, individuals will vary greatly in how they cope with their circumstances.
Some people will bow out gracefully due to medical advances, and others will suffer right until the bitter end with no relief- there is a vast spectrum of outcomes in the human experience. Thus, there's really no use in speculation. We all have our reasons for wanting to exit, and only we can decide for ourselves what is too much to bear.
I understand the frustration you feel. I've had several people say to me that I should be grateful to have painful health issues that won't outright kill me, because simply being alive is a gift and one should learn to manage with what they have, especially at my young age. There really is no ideal outcome in these situations. I hope something will bring you peace.