Imagine being able to kill the villain the first time you meet them and then having the story change and be shorter and easier because you made a snap judgment. It'd certainly make for an interesting game if they did it, but they didn't.
This is the dumbest sh*t I think I've ever heard. And I've heard a lot of dumb sh*t. I honestly don't even know where to begin.
Moving on though, this wasn't a gimmick. I know people who absolutely despised Trevor during the story, killed him off, and didn't regret the decision one bit. I almost did, because he was an annoying f*ck near the end of the game. Especially when he started to go ape sh*t after Franklin saw him trip over that small fence. The only reason I didn't kill him, is because I don't like killing off characters and I still had missions to do and money to spend. I know people who despised the sh*t out of the self-centered Michael. I don't know anyone who killed him, but if the player honestly felt like they wanted to do that, then so be it. That's why they got the choice. You had 3 choices. Simple as that. No one's asking you to choose option A or B, but they're there because different people see each character differently. Just because you didn't hate Trevor enough to want to make him burn alive, doesn't mean someone else didn't. He was an awful person, a savage. It doesn't make the story any worse because you feel that way. Someone else might have thought "Yes, I finally get to kill that f*cker Trevor", and think the story was awesome. That's why there are choices, because different people play the game. They're there for a reason.
EDIT: Take the dude above me for example. You can clearly see he's satisfied with Michael being killed off. I'm sure he loves it and doesn't think it's a gimmick. You only think it's a gimmick because you don't like it.
I'm not sure why you think the idea is dumb. It would literally be interactive storytelling. A game that changes based on how you play it. Essentially your interactions change what people you can meet and how the events can unfold down to being able to make the game story almost pointless. For example, imagine if you could kill Michael or Trevor and continue on with just the protagonist you want and see just what would happen to them. See who they met and what opportunities they got. Would Franklin ever get a place out of the hood without them or would Trevor really feel satisfied if he just killed Michael right off the bat.
A choose-your-own adventure so to speak. Games are the only place that can really do that right. Heck, look at the heists in this game. We chose who went on them based on who we found sometimes just wandering around and it changed the experience. The heist could go better or worse because of your choices. Now imagine the whole game is like that.
And yes I still think it's a gimmick to throw in three endings. I don't care if you didn't like the character and wanted them to die. I hated Lamar. Where's my ending where I get to kill Lamar? It's a gimmick because it doesn't cover all the bases and only appears at the very end without any real reason. We made no other story choice throughout the entire game and now at the very end we just pick an ending we like. It works for its intended purpose I suppose, but it's still a gimmick.