After a routine surgery, my grandfather just went downhill. He beat sepsis after losing about 60lbs because of it. Was doing great, but broke his hip, and got sepsis again after being in the hospital. Because he was so weak from the previous infection, they had him chemically sedated and on a ventilator. He was that way for about a week, and after the sepsis spread to pretty much everywhere, we as a family decided there was no way he was going to beat it a second time and let him go.
It's hard to do, but if there's no way they are going to pull through, I think it's better to let them go, instead of grounded them here and staying in a coma/sedated state for years.
It's harder, IMO, to see them hooked up to a million machines and IVs and tubes coming out and going in every orifice, than letting them go. That way you know they are out of pain.