Human behavior probably has the opposite effect in captive elephants. If a captive elephant is born without tusks then the Mahout is going to kill it rather than invest in rearing an animal that has much reduced utility and will not yield healthy offspring.

Elephants are not really bread in captivity.

They are bread in captivity. In Europe the breeding is organized by the European Endangered Species Programme.

https://nhatrangbay.net.vnwww.eaza.net/assets/Uploads/EAZA-D...

https://www.elephant.se/elephant_breeding_in_europe.php

I highly doubt those elephants are getting killed if they don't have tusks, though.

While that's generally true, there are exceptions. Elephants at the Oregon Zoo have birthed more than two dozen babies over the past several decades. https://www.oregonzoo.org/node/1948/media