I would say Unix is a system designed to be simple and powerful. It takes a lot of thinking to achieve a system which is both simple AND powerful. Whereas things like XML and Multics are powerful they can do everything but they are not simple. In the end that limits their power because they are hard to learn and things built with them are difficult to understand, because they are not simple. And when something is hard to understand it is also error-prone, and such a system is fragile.

It takes "astronauts" who think hard about the architecture to come up with systems that are simple and powerful at the same time.