Why not just make everyone a DPS?

[Reynhart]

There is no actual diminishing return as in each point of crit gives exactly the same % of crit chance, this doesn't change no matter how high you go in crit.

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.