Plex is significantly different than XBMC. Plex is a media streaming/transcoding platform and organizer. XBMC (or Kodi) is a local media player and organizer.

A properly configured Kodi setup is far more flexible than Plex and offers a lot of significant advantages (PsuedoTV, for example) as well as a larger set of supported formats and playback options.

I use both pretty extensively and the idea of trying to replace either one with the other is distasteful. Different tools for different jobs.