Pen and paper is fantastic. Phenomenally simple to use and reliable. So adaptive; you can make a mark literally anywhere on the paper, and make it any shape you can imagine. You can mix text and images with a breathtaking fluidity, with colour switching whenever you like. You can take the piece of paper and fold it, roll it, punch holes in it, spill things on it, it just keeps on going. The format hasn't changed for centuries, and won't change for centuries to come, essentially because it's near perfect - people will be able to view marking on paper, made today, for centuries to come.
You can mix and match your pens and paper without any fear of interface problems - a pen bought a century ago in Germany will work with paper made next year in Argentina. You can even pick up a piece of paper that a different pen was using, and carry on adding to the content with whatever pen or pencil you've got in your pocket. It's a complete, self-contained information display system. I could hand a piece of paper I wrote on to someone from the other side of the world and they'd be able to use it.
Anything looking to replace pen and paper is going to have to be seriously amazing. I'm genuinely astounded that people choose to make notes with Microsoft word instead. The only advantage I can see to that is that you can change what you wrote later (if that is an advantage).