Note what kinds of games populate the Next Fest. Mostly games without any other form of marketing. Anecdotally, I found four demos that interested me, and all four convinced me not to to bother keeping up with the game as it gets closer to release. EEDAR, later absorbed by NPD, the combined entity of which is now known as Circana, works with lots of big developers and publishers and found a correlation with demos losing sales. In later analysis of demos, devs found that you could (a) convince someone to buy the game, (b) convince someone that they don't like the game, (c) give someone everything they wanted from the game, where they don't want to play any more, or (d) give someone everything that they wanted from the game, where they don't need any more than what the demo provides. Note that 3 out of those 4 don't result in a sale. A trailer tended to be much better marketing material. Of course, your mileage may vary if the game's loop or selling point is hard to articulate, but in most cases, seeing someone else have a good time with a game is going to be more likely to convince you to buy a game than if you had a demo where you might not understand its appeal. It's why games are built around how well they present on Twitch these days.