They're not technically demos, though. They're full games; if you act on your own choice to play them in full, you at least actually have the ability to do so. Actual demos would not allow you to do such, you'd be forced to buy the game to enjoy more.
...which speaking of, people can still buy the GP games if they want, and do so at a discount if the game's on the GP service. Which is a neat perk imo.
Unfortunately $70 might end up being the norm, but if MS are smart they'll be able to leverage some of those games being on GP to act as a means for getting a discount on those games as long as they're on the service (not taken out of rotation). So for those games, if someone's buying them on a Series platform, they could still be getting them for $60, hell maybe even less than that depending on the specific game.
I expect that's something Sony will be forced to match in terms of an ecosystem benefit sometime in the future, but that depends on them growing PS Now, which they have a lot of work to do on that front.