First of all, I can understand your concern in regards to whether or not people would replay the demo. At the same time, though, it doesn't mean that people won't. In general, in regards to stories and story based games, there are people who read a story once and there are those who like to re-read the same story several times. Also, there are those who could replay the demo because they want to see what updates there are.
In general, what I would say is to work on your game until you get it to a point where you feel like you can upload a demo. After that, when you update things, you should write a devlog to let people know there are updates. Also, if you'd like, when the demo's available, you can write a post in threads having to do with Youtubers playing games and see if anyone is interested in playing the demo. Also, there have been times when Youtubers have replayed demos when there have been updates.