Its a mistake for PDOX to release a demo. They will only lose customers who do not like the game. Its better for them to make everyone pay the price and then uninstall if they do not like EU4. I uninstall the vast majority of video games I buy within 2 weeks. If they offered demos, I would have failed to buy all those games.
I for one would very much like to try the demo just to see how it runs on my PC. If it runs ok, I'll immediately pre-order the game.