Right...well, maybe you'd be extending the life of someone who makes everyone around them happy. My cousin, for example. She is possibly the most caring, loving, humorous, and positive thinking person I have ever known. She always has a funny story to tell, and always makes sure that no one is left out of the conversation. Smart as hell, too..she was always able to figure everything out for herself, even as a toddler.
She is also 28 years old, and dying. She is in the later stages of cystic fibrosis, and desperately needs a double lung transplant. She is low priority, because there aren't many lungs to go around, and the likelihood of her getting an infection afterwards is high. Perhaps if there were more lungs out there, the doctors would be more willing to take the risk. She is on steroids, and currently walks around with a tube in her nose, because she cannot breathe well enough on her own.
So think of people like her. You're not only denying her more life...you're denying everyone around her the pleasure of having her in their lives. The world would be a better place with more people like her around...not less.