agent x: Dont count on it. How many people say 'im never flying with xyz ever again so there!!' when something doesn't go their way and its usually..as in this case...the fault of the passenger i.e not having an ounce of self initiative. AC wont be interested or bothered that they are gonna lose a handfull of cheap tickets from them...you can count on that.
I agree with that. And a long letter is less likely to be read properly by customer relations people than a short one.
However, a letter that might have an impact is one that says something like "Unfortunately, I fell foul of this particular situation (described), and I have a suggestion for improving matters in the future: Queue combers should state the intermediate destination [ZRH] as well as the final destination [DEL] in case there are other pax who, like me, didn't know the final destination of my flight."
"Might", not "will", but it seems to me to be the only type of letter that could produce a positive response, even if it is only a sincere apology rather than a standard form letter.