You two are getting predictable
First of all, in BoVD the discipline prestige gives a triple treat area, stacking with improved critical and not with keen. Clearly, they intended it to work that way, even if it is 3.0 and not 3.5.
Other than that, that ''unlimited attack chain'' isn't so ''unlimited''.
Supposing that you get an extra attack for every threat,
The possibility to get an
extra attack
x times would be
1:60%
2:36%
3:21%
4:13%
5:7%
6:4%
7:2%
So most of the times it wouldn't go that far.
Also, in p140 of the PH it says that if you have an increased crit range, a roll within that increased range it is not an automatic hit unless it is a 20. And that if it won't hit, it wont be a threat. So the chain will probably stop earlier.
So against a low AC the 9-20 range may give you an easy kill if you are lucky, but against higher AC its pretty much the same as a normal crit range would be.