I don't see what's wrong with "playing god." I know for a fact that the Flying Spaghetti Monster (may his appendages always remain noodly) doesn't care if we mess around with DNA. I think in fact he likes to "play god" right along with us.
Essentially, people are uncomfortable with change. The people who would believe this is wrong are those who draw some sort of murky line between radiological, chemical and surgical medicine we have now, and the genetic medicine proposed here. Unfortunately for them, that's a rather indefensible position in the long term. If we could make everyone live to 300 years old, there would be people who would draw the line at what a "natural" life is supposed to be and argue that it's "playing god." Never mind that with medicine basically everyone's trying to live as long as they can. It's like: "Oh, I wanted to be successful, but not THAT successful!" However, asked how long they want to live exactly... they probably couldn't give you a consistent answer as they progress to that age...