Very true. The person writing the questions has the advantage in this one because they can always throw in more variations and I would have to figure out all of the new ones.
There is, however, an inherit limitation to question-asking, and that is it must be understandable to a general population. So I would switch from using a parser to just looking for key words and then matching from there. (Which is, for example, how I find names in a list.)
Even if they made it so I only get one fifth the number of correct answers, a 20% success ratio would be high enough for concern. The attackers can just keep trying and trying and if 20% of the time they can register a new bot, then they win. The defender must win a much larger percent of the time.
Aircraft fly through the? A: FAILURE
Q: A beetle has how many legs? A: a
It does well when it recognizes the questions, but there's still plenty of room for writing new ones. It does show that it's an arms race, as someone else noted.