I don't agree that if people can get something for free they will always do so, especially not when they are capable of buying it and have an interest in buying it. If everyone did that all the time, no one would be launching SP games on PC, X360 and PS3 anymore.
The rest of the stuff, nothing new to what I've already said. Either people 1) genuinely buy the game, 2) buy a used copy of it, 3) pirate it, or 4) do nothing. Unless someone had the capacity or interest to pour money into the game, cases 2, 3 and 4 will change absolutely nothing for the developer. Oh, and it's true that for a used game to exist, it should be bought in retail first. I agree that piracy in this matter is just a lot easier.
I'm very well aware about how the livelihood of a game company is tied down to the sales earned by the game. But again, how much of a possibility is there for a pirate to actually go out and buy the game if they were somehow magically blocked from being able to pirate it? In my opinion, very very little, because most likely they never had the intention/interest of wanting to legally own a copy of it. So either they get the game illegally or don't get it. The sales of the game and the status of the developer changes in no way. And when I say pirates, I'm not just talking about the infamous ****** groups but also all the others who download the games.
As for DRM Vs no DRM. Games with the most annoying versions of DRM and games on GOG are equally pirated, which is why I'm against DRM. The only difference is that the crackers don't have to crack the game when it comes to GOG. All DRM does is annoy genuine game owners from accessing the game content right away/when they are offline. At times, DRM even drives people away from buying it like Ubisoft's Uplay.