It's not as if being overweight or negative health consequences of smoking has much bearing in D&D. These are things that may have less to do with their Constitution score but their lifestyle choices. In D&D, these are often relegated to aesthetics. You can make a grossly overweight character with an 18 Constitution. You can say that they are chain-smokers and they would still have a 18 Constitution as far as the game is concerned because D&D is not concerned with simulating these things via their ability scores.
So how could this work though in a system where Strength and Constitution are combined? Training and Skills. If we look at Forbidden Lands, which Yaarel mentioned several posts earlier, it combines Strength and Constitution under Strength, but it distinguishes between Endurance and Might for skills.
I disagree because otherwise what
do these things represent?
You could not have a CON 18 (well, 20 in 5E) being grossly overweight (heart attack, anyone?) and a heavy smoker (lung cancer, no wind to keep swinging your sword repeatedly, etc.). CON is your "health, stamina, and vital force" in 5E, admittedly a simple definition compared to other editions.
While these things have no mechanical "bearing" in D&D (saying your character smokes 20 pipes a day for instance), by the meaning of CON you should consider what than means. Could a heavy smoker have a 14 or even 16, maybe if their other elements of health (low body fat, good diet and genes, physical activity to keep them in "shape", etc.) were all good. Could a very heavy character have a good CON like 14 or 16 also? Sure, if other things are exceptional, they are off-setting the body fat enough to raise the average to high levels.
One of our players is massively overweight at 29 y.o. He has joint issues already, moves slowly coming down the stairs, etc. Now, his "vital force" might be great, at least a 5 out of 6, as he is full of energy and always alert, etc. But, his health is probably a 2 or 3 at best (out of 6), and his stamina maybe a 3. I could see his overall CON being 10 or 11 (average) at best, being generous you could even argue for a 12. If you go to the low end, he might only have a 9 (I doubt lower, though). If he lost weight, exercised and made better food choices, I could see him moving up to CON 14, maybe even higher if he really kept at it.
Another player is younger with very low body fat and somewhat active (works outdoors some), but smokes a lot. He can be active for a while, but gets winded. His diet is also not very good. He is probably sitting on a CON of 12 or 13, maybe 14 at best.
Personally, my own CON is probably 12-13 as well. I am older (45), but never smoked, rarely drink, somewhat physically active, eat a decent diet, but I am overweight. If I lost 30-40 lbs, I would probably have a CON 14-15. If I started working out a lot and improved my diet even more, I might make a CON 16 even. That's it for me, though. I would have to make major lifestyle changes, etc. to ever get better than that, and honestly might not have the genetics to even go higher. But, that is why I say when I was just starting college, running track (sprints, hurdles, jumps), cross country, lifting weights 3x weekly, with about 10% body fat, I could easily argue for a CON 14 -16 even.
If a CON 20 is someone with perfect (well, as best as possible) health, stamina, and vital force, I would easily subtract a point or two for each issue they have. So, you argue for one issue (overweight, poor diet, bad genes, smokes, drinks excessively, etc.) an 18 or 19 might be possible if you play as if all other things are excellent, but throw in more problems, and CON would go down.
After all, when someone plays a CON 8, what does that character often look like? Sickly, small framed, often tired, or a similar condition.
People complain about dump stats. I very rarely have a character without 10 or better in everything. Once in a while I might choose a 9 for something if I then play it off as part of their make up. An elf wizard with a STR 9, an overweight cleric with DEX 9 or CON 9, a shy and uncertain monk fresh out of the monastery with CHA 9, and so on.
It isn't as though you couldn't represent everything as skills instead, but the core 6 work for me because I have a clear and distinct understanding of what the mean, how they work, and there is very little overlap. Are strong characters more likely to have good health, sure, of course, but it isn't unreasonable to have a STR 18 and CON 10 or 12 or vice versa. I wouldn't want a system personally where I had to have dozens of skills or feats to compensate for all the variety of combinations I can have by just using the core 6 ability scores.
Sacred cows has
nothing to do with it for me. If someone comes up with a better system, I am all for it.