I guess I can understand the idea that the higher incidence of lung cancer in smokers, for example, could, in theory, be due to some other variable, it seems to me that the cause/effect of smoking clogging up lungs with tar is a well proven fact evidenced by millions of chest x-rays. And I suspect it would be equally difficult to dissociate the tight tie in between smoking and COPD (related to those clogged up lungs). So I am curious about your take on this.
1 - I would say in reality that the incidents of lung cancer in (some) smokers is due to many variables. I would see it as (true) cause and effect if all smokers who smoked x amount had lung cancer. Short of that, I cannot call it cause/effect, but can understand the association or correlation factors. I would hope with that understanding would come understanding that other variables likely play a role in incidences observed. Not maybe / possibly, but likely.
2 - My take on the effects associated with lung issues and smoking is that there are variables at work besides smoking and that abusive smoking (heavy use) would appear to be a significant factor in most, if not all, known cases of a smoker who has been diagnosed with some sort of reduction in lung functioning. But not the only factor and ought not be presented as only factor, ever. To establish that, I would require hard science to do such testing. If deemed unethical, then we will never know and can never then assert that we know based on statistical analysis. But again, the abusive factor must be clearly stipulated in what's being presented as to how one arrived at point of reduced lung function. Thus not (mere) smoking, but more accurately abusive smoking, as in smoking too much.
3 - Prior to my vaping experiences and being one that had experienced abusive smoking (for around 10 years) and then cold turkey (for around 10 years), I noticed that it is possible to regain lung function. Perhaps not at 100%, but for sure better than whatever it is abusive smoking does. My opinions then were smoking is very bad for you. I even had some (albeit small) hatred reserved for BT. Since vaping, and becoming aware of political stuff, I no longer see it as BT issue. The politics over the years had BT playing a game that they willingly participated in and did manipulate things, but is challenging to see them as pulling the strings / outright lying. I won't deny they probably did at some point, but will dispute that they have been lying to their consumer base. For me, though the main point of this 3rd item is that I have now become moderate smoker. I would say that 20 years ago, I saw no way of how that could be done (given how cigarettes seem so highly addictive). I had seen others who did smoke moderately, but never understood how that was possible. Up to 3 years ago, it was a mystery to me. Within the last year, I have become that, and thus the mystery is gone. With vaping in the picture (for all to partake in), I'm now not sure how abusive smoking is entirely likely. I think moderate smoking is now available to anyone who wishes to go that route, and I respect those who either don't wish to smoke ever again or rather only smoke, as is their recreational choice. But my bias is that recreational, moderate smoking does not kill and I'm very much wondering if it even harms. That it might harm a little bit, I won't dispute or if I do, it'll amount to me claiming at some point that tap water will harm a person, a teeny tiny amount. Why debate the small stuff? I believe moderate smoking is now able to turn the whole "smoking kills" meme on its ear and send it to the place where gross lies go to die.
I'm a 40 year now ex smoker, thanks to vaping. I am as cynical as anyone as to statistical correlations. The old saying... liars figure and figures lie. But I have no doubt whatsoever what 40 years of admittedly very heavy smoking (2.5+ PAD) did to my lungs. And although I feel "1000% better" after 6 months smoke free, It is also obvious to me that I will never regain the lung capacity I had even 15 years ago.
Given what I wrote above, and given your stipulation of "very heavy smoking" for 4+ decades, I see no disagreement that we might have.
I would just note the fact that vapers (now ex-smokers) can recover aspects of their health, in a way that is meaningful to them, in such a short period of time, strikes me as very encouraging news. When you go cold turkey, there is the psychology involved that will at times doubt the path you are on (and think a cigarette will somehow magically make things all better). With vaping, those doubts go way down and the alternative path doesn't make you want a cigarette or think it will make things all better. I think that matters in the scheme of things, or in reality. But looking at things strictly from clinical perspective and in theory of "what is best," the cold turkey route does strike me as most promising path (toward better health). Yet, psychological factors being what they are, I'm not so certain about that.