I get this a lot from my brother (Geneticist turned Doctor) and many of my "smart" friends (actuaries, physicists, surgeons, biotech engineers, programmers and mining engineers)
"Finance is NOT complex, all you need to know is:
NPV/IRR; High Risk -> High return; Buy low, Sell high"
"All the other stuff you'll learn as you do - Petro-chemical engineering, becoming president, human stem-cell organ replication, ancient hieroglyphics, particle physics, understanding madarin and arabic, genetic algorithms, bettering Shakespeare, and signal processing with Kalman filters, giving birth is complex; not this finance stuff."
"I can easily replace you at the bank"
Please provide me with some good ammunition for retort!