That's a tough call. As only a grand child I would imagine you don't have as much of a voice. We went through a long and drawn out death with all 4 of my grandparents. It was tough.
The best advice I could give you is to not let your family make the Decision on convenience. Not saying they would, or they are, but it can be an issue for some people. "We don't want to have to watch after mom or dad for 4 or 5 years, lets "end their suffering". Like I said, I don't think that's the case here or very common, but it happens.
From the Christian Stand point, I would hope your Grandpa would have realized by now that he is not in control of his life. God is. And while God does have his timing (by using modern medicine) it might not be for purely your Grandpa's life. God uses our Sufferings to make us stronger. He could very well have your Grandpa in the position he is in to make an impact on someone he interacts with on a daily basis. Coherent or not.
If it were me, I'd make sure he was comfortable and wait for nature to take it's course. That's how my parents (and uncles and aunts) handled all of my grandparents.
And just remember, they took care of you when you couldn't talk, you pooped your pants, and you weren't able to coherently talk. Seems only far that the favor be returned.