To be honest, when I read something like "free demo" I skip it automatically. Not because I'm ignorant, I just don't want to attach to something that I can't have at the moment. Also, I think demos aren't really working in the way people plan it, it's mostly because of my perception, but there are a dozen other visual novels with completely free updates, still having more followers on patreon. It's really about to find the middle golden way, so every party is happy. I would also rather give my money to someone who have a "writer-reader bond", like not being cold with updates, it would actually boost my moral for support.
I was also thinking about it when I worked on my own visual novel like years before and I would have also stick with the free updates. (Only if I had the ability to share it) Creating a "bond" with everyone instead of only with patreon members would cut the audience and other possibilities short. "Poor" people can be future supporters too at the end of the day.