Why not just make everyone a DPS?

[Phoenicia]

That's exatcly why it's considered a "diminishing return".
If you need 100 for 5%, going from 100 to 200 actually doubles your ctitical hit rate while going from 200 to 300 only increase you chance by 50%.
It's also why Skillspeed/Spellspeed have an increasing returns since 0.1s weights "more" when you have 2.0 GCD than when you have 2.5.

Yeah, that doesn't change the linear nature of how stats increase per point. It just means the increase is linear but the benefit is "diminishing".

Every stat increase increases damage by a less % of the point before. (When you have 100, increasing it by 1 is 1%, but when you have 200, increasing it by 1 is only 0.5%). This is true for all stats with the exception of Skill/Spell Speed.

The "diminishing returns" I was thinking of was that stats increase with a formula with a denominator. For example in League of Legends, armor decreases the damage at a rate of "armor / (100+armor)". Each point reduces damage less than the previous. You get a non-linear (it's a curve opening upwards) graph that closes to, but never reaching, 0. Though the benefit is linear (each point increases eHP by 1%), the effect of each point is "diminishing". At 1 you reduce damage by 1%, at 2 you reduce damage by 1.96% (each point averaged 0.98) and at 3 you reduce damage by 2.91% (average 0.97) and so on. At 100 armor you reduce damage by 50% (each point averaged 0.5%) increasing eHP by 100%. This made it so it is equally important to build HP as well as armor/magic resist to efficiently increase survivability as you had only 6 slots for items.

With disregard of semantics. My whole point is, in here, you end up with a linear approach of "More x stat is always better" and you have no point where you should "stop" building that stat in favor of another stat for added efficiency. Or if that point exists, it is far beyond our gear's reach (Current and future patches').