I have a problem with the word reusable. A reusable software should be a component in a larger solution. Not a complete solution itself.

If I accept that Stack Overflow or Discourse are reusable, because you can run them on different urls with different graphics, topic, moderators and users, then every application, for example Photoshop, is reusable in the same way, because multiple people use it to perform multiple tasks with images.

What you do with Discourse is good engineering, but there is nothing special about it. Every serious CMS, like WordPress or Drupal must inevitably be usable more than once.