This is like saying old software is so simple that updating a line of code can break an entire application. It's a silly thing to say. No matter how complex or how simple a piece of software is, you can easily break it. If you have a program that prints out "hello world", guess what? Updating a single character can break the entire application!
The world is more complex now. We've stood on the shoulders of giants who stood on the shoulders of giants. A few centuries ago a renaissance man could make advances in multiple fields. Now people are specialized. It's the same thing with software. Of course, people take it to an extreme. However, you go ahead and write your own crypto library, I'll use a vetted one created by experts.