I have never read someone using the phrase “chd stacks with elemental damage” or “skill damage stacks with mainstat” or “elemental damage stack with some set bonus”. That’s not what “stacking” means in D3.
Whenever the game guide speaks of “This effect stacks up to x times”, the bonus effect is added x-times, not multiplied x-times. For example the skill Soul Harvester. Which each stack, you get 3% more Int, up to a maximum of 15% more Int. That’s 3% * 5, not 3%^5. Or take Gogok, BotS, Taeguk … whenever we talk about “stacks” in D3, the different values on this stack are added together, not multiplied.
So why start using “stacking” with two different meanings?
Now you’re implying that some well-rolled legendary items with a good amount of chd, skill-damage, elemental-damage, mainstat and so on are better than a badly rolled class set. Again, that’s not true. (I do understand what you’re trying to say. But the “single massive stat” is often way better than the product of all the “regular stats”, because it’s - well- “massive”. 
)
That’s true, but it is mostly irrelevant, because you can never choose on a item between Skill and Elemental damage. However, the math behind this affects choices like “Mainstat or elemental damage on amulet”, or “skill damage or CHC on helms”, but it’s a little bit more complicated there, as the numbers have way different values in these cases.
Set bonuses do not push us “a few” GR levels. They teleport us into completely different GR-spheres. The sets are buffed so massively over time, because Blizzard had the philosophy “never nerf, always buff” when it comes to balancing. Through that, they started a power-creep-spiral, which spins faster and faster and lead us to completely absurd buffs.