If we really must discuss this, which is self explanatory and incredibly obvious, the best way to look at it is to ask how many DAMAGE points I mitigate with an extra PRR.
Or if we can just compare these two options, 200 mitigation give me (200-67)/200 = 0.65 damage mitigation per 1 PRR.
And the second option, 300 PRR gives me (200-50)/300 = 0.5 damage mitigation per 1 PRR.
It is obvious there are decreasing returns to PRR. It follows from the mathematical definition of "decreasing returns" which just means that the second derivative of the PRR function is negative. Damage mitigation is a concave function in PRR.
There is no need to discuss over these details. Any player can look at a table and see:
Oh look, going from 200 to 300 PRR makes me take this % of extra mitigation. Is it worth it?
The discussion is whether the proposed change is working as intended and to that I have to give a very firm NO.