The net damage is the same. 2 more hits every 100 rounds is always 2 hits worth of damage. if you do 10 damage a swing and get 2 more, you always do 20 more damage. That is true.
The RELATIVE gains are not. If you increase your damage by 20 for every 100 you do, you do 120 damage instead of 100. Your total damage output increased by 20%. If you do 200 damage, but still just increase by 20, going from 200 to 220 damage is only 10%. That is diminishing returns. The actual net damage increase may be the same. Both add 20 damage. But if your base is far higher, then it will only make up a relatively smaller chunk of your TOTAL damage.
It really doesnt technically matter how you measure it or what you call it to determine the end result of what gear/atma/buffs make you do the most damage. Both statements are true. However, you MUST COMPARE in the same way to the alternative gear/atma/food/buff/whatever. If a pole grip gives you 20 damage and a brave grip gets you 15 damage. The pole is better. If pole increased damage by 2% and brave by 1.5% its the same thing. It doesnt matter if you say its 'diminishing returns' or not. So long as you calculate the damage properly the terminology is irrelivant aside from consistency and clarity in communicating ideas to others.
Conceptually, adding more of any stat aside from haste reduces the % gain of each point after the 1st by more and more until said stat caps (hard or soft cap) because you are adding the same net damage with each increment, but your overall damage is increasing making additional amounts of the stat a smaller and smaller chunk of your total damage as you progress, even though each point adds the same damage per time increment. Its just a numerator and denominator fractional relationship.
Please do yourself a favor and look up what "Diminishing Returns" actually means, without listening to what other Alla posters are telling you. It is simply not what you are talking about.
You are arguing percentage of increase, which is not at all the same thing. Sure, percentage-wise, each time you add a static amount, it's going to be a smaller percentage increase. If every year, you get a $0.50 pay raise, you are getting a linear increase. Percentage-wise, it is a smaller increase, but does that make the slightest bit of difference? You still get the same benefit from it. (ignore taxes for this discussion)
If you take all the math you've done and call it "Smaller Percentage of Increase" then you'll be on the right track. It will also help you make better decisions going forward.
What I am curious about is, do you also apply the same logic in reverse regarding haste? If there is no such thing as diminishing returns, then haste should not have increasing returns either as they are just opposite sides of the same coin. You cant accept increasing returns on haste while disregarding diminishing returns on acc/atk/DA/TA/Fstr/WSC/FTP and all that jazz.
Haste is increasing returns. When you graph it, your benefits from haste rise in a parabola shape (much like the right half of a U).
If you were to graph decreasing returns, it would look like a U rolled on its side, facing the right. There would be a sharp rise at first, followed by leveling out, and eventually virtually no rise at all.
DA's graph does not look like that. It is an angled line that is straight and consistent.