Rather than attack, why not send them a link to this thread and allow them to step up and make the righteous decision?
Possibly you communicated with someone new, or who only feels confident enough to follow procedure. Almost EVERYTHING is negotiable in life, and certainly this is.
Thank the person on the phone for their help and say you understand and respect their position, however would like to speak with a supervisor to discuss this further. Every time I've ever done this I've gotten a different answer, except for with cops of course but that doesn't stop me from telling them *** I think of their BS.

Sometimes that's all the satisfaction you'll get, but it's worth the effort and just miiiight make them think twice and make a change down the line.
Alternately, maybe someone can explain their position and it turns out to be defensible. It's not about it being one month out of warranty, it's that there are thousands of these basses that have been played daily for 30 years or more and never failed so the preamp "should" be covered for life. However, maybe they have insurance against, say, mass component failure like a bad batch of op-amps, and that insurance demands they limit liability.
Who knows unless you ask?