Your concern here is exactly why demos don’t typically exist anymore. Apparently they make more money from marketing hype than roping anyone in from demos, and demos have a tendency to turn just as many people off as they lure in. Besides that, it can be difficult to create a slice of a game which represents the whole, especially for games with any amount of complexity. If you put a player too close to the opening of the game it may not have all the cool powers and toys available to make it interesting, but if you put them in a later scene it may be overwhelming to play since they didn’t get to learn how to play step by step. A lot of times it costs development time and money to make the demo too, so there’s an investment there that needs to see a return. When players are willing to pay for early access and essentially do the bug testing for you why would you make a free demo and lose customers?

Steam next fest is a little different though, since it largely is a marketing festival, highlighting many games which likely dont have much of an advertising budget. In this way it serves a purpose by getting people talking about games they otherwise would have passed by, so I think in this specific situation the demos are very worthwhile for the developers putting them out. I personally like it too, because sometimes looking at the landscape of indies it gets a little muddy trying to determine what’s going to be fun or not just by looking at it. I will always take a chance for a free trial of something interesting to me, and if I find something I like I will tell my friends to give it a shot. It would be interesting to see how effective the festivals are, but considering they keep doing it they must be seeing an overall positive impact on sales.