but the best money is made with release and preorder sales.
I think this type of "system" works best for consoles, but for PC it's a different story ... while a console game, in order to return profit, needs to sell millions of copies (do to high cost of development) in a short timeline, PC game returns profit to devs after months and even years after the game hit the stores and even then, the PC sales rarely reach 1M mark, but if it does (even if it sells 500k copies), it is mayor success for devs and publishers... ofcourse there are exceptions, like Blizz titles (WoW, SC, Diablo) and EA titles (like Sims)
As for demo, personally, I don't need one, just focus on full game and everything will be 'allrite
