Just a thought:
Fey creatures cannot break their word. Not, I point out, they cannot choose to break their word, but they can't do it. Presumably, this means that if you force a fey creature into a bargain it can't fulfill, it becomes something other than itself, and definitely not fey (presumably, a dead fey).
Let's say that the only circumstances that would result in J. Random Mortal managing to talk a fey noble into an insolvable bargain are a +4 / -4 shift on the fudge dice. That will happen 0.01% of the time, or once out of every 10,000 encounters.
Now, let's say that on average, one person a year is insane enough to throw down with a noble fey. That means that you'd expect fey to last, on average, about ten thousand years.
Is there anything backing me up on this? We know that there are powerful, powerful fey who don't seem to want much to do with humanity; might this be because they are very aware that mortals might just invent Lojban and start a conversation with them in it?