Re-writing code that nobody understands and hoping that it will work correctly is just wishful thinking. If people really wanted to understand the code, they could do the hard work of reading it, tracing it, writing tests for it, etc. In many cases, you can transform the code into something readable by applying a long series of simple refactorings.

For some code, breaking behavior you don't understand doesn't make a difference. If you're Facebook, you can arbitrarily change the user interface of your site and your users have no say in the matter, since they're not paying customers. But other developers don't have it that easy. If you support code that people have built their own applications on top of, you can't just break stuff. If you make backward incompatible changes to the Linux kernel APIs, or break Microsoft Excel so that macros that have been working for years stop working, people all over the world will be very unhappy.