Show them the Beyonce Rule part of the Google SWE book.

We are often asked, when coaching new hires, which behaviors or properties actually need to be tested? The straightforward answer is: test everything that you don’t want to break. In other words, if you want to be confident that a system exhibits a particular behavior, the only way to be sure it will is to write an automated test for it. This includes all of the usual suspects like testing performance, behavioral correctness, accessibility, and security. It also includes less obvious properties like testing how a system handles failure.

We have a name for this general philosophy: we call it the Beyoncé Rule. Succinctly, it can be stated as follows: “If you liked it, then you shoulda put a test on it.” The Beyoncé Rule is often invoked by infrastructure teams that are responsible for making changes across the entire codebase. If unrelated infrastructure changes pass all of your tests but still break your team’s product, you are on the hook for fixing it and adding the additional tests.