Is it mathematically true that as you gain more and more crit, every new point of crit will be cannibalizing previous points of crit to a larger and larger degree?
No, not in general. Certain class mechanics can cause an effect like that sometimes. Here is an example to help:
BigFacePunch does 100,000 damage, 200,000 on crit.
I do BigFacePunch 100 times in a fight with 0% crit, that will result in an average damage of 100,000.
If I add 10% crit to my gear, the damage will now be 110,000 on average.
If I add another 10% crit to my gear, the damage will now be 120,000 on average, an increase of 20% from the base.
Adding crit always yields the same damage increase, there is no diminishing return.
People will sometimes say it is worth less and less because increasing your damage by 10,000 when you start at 100k is a 10% increase. Increasing it by 10,000 when you are at 110k is a 9.1% increase. So, the same amount of crit will increase your DPS by the same amount, but, that amount will be less and less of a percentage of your total DPS.
It’s really an argument over terminology. The value of crit doesn’t diminish the more you get.
ok, that is helpful thank you.
the reason I ask is because I use a baseline of 10 dps in the AMR sim tool for my crit and then base the other stats around that. 10 for crit, 8 for haste, 7 for agility and Versatility and 6 for mastery. Is this dead on accurate - no, but its correct from a scaling point of view and gives me a generally accurate idea on gear rankings. My question on crit cannibalization came from the perspective of well I am already at 51% crit and 20% haste. I wonder if it is worth it to go to 47% crit and 24% haste… hmm
Stat weights can’t really answer that question, which is why we created our new method for picking gear, which uses a more complicated formula.
We create a graph like this:
http://www.askmrrobot.com/wow/simulator/report/5cd36127e908450c8c755fc32ef877f5?weights=true
That helps you see the answer to the exact question that you are asking: Should I keep getting a stat, or start to balance it with another?
ok thank you.
at first I did not understand that graph other than to think “wow… pretty colors… clearly its a nice picture that means nothing… I guess I will just scroll down.”
lol… but your explanation and some time combing through it helps. Its too bad that the answer for Havoc DH’s appears to be… get as much crit as you possibly can at the expense of everything… at least its an easy answer 
its a good thing that my livelihood does not depend on my statistical analytics skilz eh?
p.s. it does 