Demo's have been replaced by Let's Play. If they had a made a great game people would buy it. This is a niche game that really isn't going to be most people's cup of tea at $15. Similar games with a wider appeal have done quite well despite the existence of Let's Play. Dragon just isn't a compelling product at this price point for most consumers. I liked Dear Esther, The Stanley Parable and Gone Home which all shared a rail road narrative style. Good games that are priced right do better for Lets Play. There are so many games competing for attention and sales Lets Play is a great way to figure out what's worth my time and money. I just picked up Caves of Qud when it went on sale after watching a few Let's Plays and I'm loving it.
I've seen this title a lot on the Let's Play channels I watch. I'm not a gamer, but if I were I'd consider getting The Stanley Parable and The Beginner's Guide, even having seen most of it on Let's Plays. I don't think that having the game spoiled on Let's Plays deters people from getting it. Although I haven't watched any of it, the word cancer in the title tells me that it's probably morbid. Some people might like their entertainment to be sad and difficult to watch, but I'm not one of them. The developers might consider that their subject might not appeal to everyone.