I think we need to clarify a "demo" here, as well.
A traditional demo in gaming is often simply a mission, or the first act of the game. That is to say, its the final product just restricted. The most classic example is the 90s shareware boom. Where you'd get Act 1/3 of games, then would have to pay for the rest if you wanted to finish (This was a staple of Apogee-later-3d Realms, and ID software in particular. For instance, DOOM (the original 1993) was released with Act 1 (nine levels) as a demo or shareware, with Acts 2 and 3 being in the purchased game.
While the various preview/stress test/early access/beta weekend stuff in current days would technically act as a preview, they're often not indicative of the actual game. And in some cases, fraught with technical issues. Monster Hunter Worlds early preview weekends for instance, didn't incldue the town hub, any aspect of gear customization, or progression. It was just "heres you dude, here's 3 hunts you can do".. Which certainly is a poor impression when compared to the overall game. To swing the opposite direction, Anthem fired you on (to the best of my knowledge) the first few missions of the game during its previews, but was a completely unplayable mess due to various bugs, but notably a frequent infinite loading screen, which probably killed off interest and dissuaded potential buyers more then any positive gains.