In Math, if you say "DPS isn't linear in crit rate (the correct term should be affine, but let's forget about it)" is wrong. You specified a variable (crit) and a function (DPS) and the last formula is (for the part affected by crit, but you add a constant for the part not affected by crit) DPS = baseDPS * (2+0.0697*crit–18.437)/2 and this is a line, i.e an affine function (linear if you want). Now, since you specified CRIT ad the variable and DPS as the result, no we shouldn't understand "increase of DPS in %". In this case, the situation is a little more complicated : you have 2 variables (old crit O and increase in crit I) and the formula is : %DPS = DPS(O+I)/DPS(O) - 1. this function is of course affine in I but not in O, and that's what you are calling "diminushing return" : for a same increase I, the answer is a decreasing function of O.

If I ask you, durng an exam "is DPS linear in crit" and your answer is no, you failed the question.