Together, they produced a strong base design and an integrated framework for extensibility and longevity. Of course, these aren't the only reasons that X is still popular. X did fill a need at just the right time. Market forces along the way didn't hurt. Still, the design lessons are important to learn.
Good designs can greatly improve the success of any software project.
I encourage you to study the X Window System's design history and look for ways to apply these lessons to your projects.