When SE introduced the rare/ex versions of emp.hairpin and leaping boots, they didn't suddenly change the pre-existing ones to rare/ex. In fact, the pre-existing ones are still relatively high priced because for some people it's more convenient to spend the money than it is to camp the NM.
Thief's knife should have been no different. They could have put the non-rare/ex one in a bcnm (if they decided to allow for more non-rare/ex ones to be introduced at all) and then have the new rare/ex ones drop from the nm.
But even if they put a non-rare/ex one in a bcnm, it still shouldn't have nearly as horrid of a drop rate as k.club and pcc.
I'm just tossing out ideas. In the end, it's just more woulda-shoulda-coulda. But it still seems like a few good ideas to me.