I often wonder this about my dad and if he had have been around a bit longer had he never got diagnose, he was unwell and not quite right but no impact on his day to day life was still doing the odd job erecting a fence and lifting and relaying a driveway, once he diagnose he went downhill rapidly and lived only 5 weeks from diagnosis once they start treatment.
In your dad's case, I wonder if it was toxicity of the treatment that caused his rapid decline in health, or if his mental state took away his will to live, or if it was some combination.
I can't speak for chemotherapy, but it's almost a cliché at this point that the chemotherapy drugs themselves are worse for some people than the actual cancer is. On the mental aspect of being diagnosed with cancer, I wonder if a diagnosis becomes a self-fulfilling prophecy for a lot of people, and with that - if people just quit on life. Basically being defeated, and allowing cancer to overtake and defeat them as a result of a mindset that they are going to die anyway. I wrote "a lot" of people rather than "some", because it seems to me that people are pretty negative regarding health matters that they feel they have no control of - Covid-19 has made me see that.
I'm sure that it's an oversimplification to say that negative mindset results in people diagnosed with cancer dying faster, but I suspect that a lot of people do adopt a negative mindset upon diagnosis, and that some portion of those people end up dying sooner than they otherwise would have, because of it.
When I look around at all the people worried about Covid-19 right now while I literally could not care less than I currently do about this virus - and HOPE I get it, so that my immune system can kick its ass, and I can help human beings develop herd immunity against it - it just makes me wonder how much of our health outcomes are impacted by negative mental states.
I see numerous people who are extremely overweight or who smoke cigarettes [or both] who are wearing masks. These people are so scared of a virus that they have little to no control over ultimately getting, yet don't seem concerned about matters of health that are very much within their control to improve upon [i.e., lose weight by reducing calories, or reducing or quitting cigarette smoking].
Mel Gibson was on Joe Rogan in early 2018, when his dad was 99 [literally seven months before his dad turned 100 - and his dad only died this past May, three months shy of his 102nd birthday]. One thing Mel said on the JRE podcast was that when his dad went to get treatment in Panama, the doctor couldn't believe that he was 92 years old and not on any medications. When he asked how that was possible, Mel's dad said "I never go to doctors."
It just makes me wonder...if things are going well, and you aren't in any severe pain, does it even BENEFIT us to find out that we have the small start of cancer growth? Or some minor blood pressure issue that we end up getting medicated over, with a medication that may well cause any number of other side effects or problems? I think in my case, I would just fall into a pit of hope and despair, and immediately become defeated, and let the cancer beat me. Meanwhile, if I just don't go to doctors from now on, but continue to exercise and eat a low calorie healthy diet and live an excellent lifestyle, I will probably be fine - without going to a dark place in my mind where I think I'm going to rapidly go downhill and die, much the way you explained how things went for your dad.
I don't know...it's just a thought...but let's just say that while modern Western
medical care technology does benefit a lot of people, I think that with things like cancer, so much of it is, sadly, genetic, and I am confident that some people would have been better off if they simply didn't even find out they had cancer at all. But who knows...maybe I'm way off here. I guess the bottom line is, human beings have not managed to beat cancer yet, and that's just the way it is.
Here is the video with Mel Gibson, talking about his father and the stem cell treatment he got in Panama when he was 92 [I think in 2011]. He lived almost a full decade after that, only passing away last May - so looks like he picked the right time to seek medical treatment. 
