Rails plugins are different from Discourse plugins due to many reasons. Discourse plugins need to have a specific folder structure, they can extend/override the ember code of the core discourse etc. I think the rails side of a discourse plugin would be closer to a generic rails plugin but they aren’t the same things.
Moreover, one should only create new tables if they’re really needed. Discourse already has ~150 tables and its not too often that your job isn’t done using one or a couple of them.
But, moving forward, I personally would wish and like to see that happen. It would mean people are building huge and more exciting plugins and getting more creative with what could be done with Discourse.