Altering our DNA only seems like a good idea on the surface. Sure they talk about obesity genes and disease genes and all that, but our species has evolved on its own to meet the requirements of our environment. And part of that evolution is we are among the least adapted species meaning we already can survive in a variety of niches without threat of extinction. When we start messing with our coding we are creating artificial adaptations, that cause us to be more adapted to a specific niche or conversely opens the modified human to harm or an attack or death from something most unmodified humans take for granted.
Take something like a GM Crop that produces a toxin to fight Pest A, in doing so it might attract more attention from Pest B, or result elimination of the deaths of the weaker plants, which Pest A usually ate, which allows the weaker to breed in a genetic weakness that reduces crop yields significantly.
Say we introduce a new gender chromosone into humanity one new chromosone could introduce three new genders with a new variety of biological disorders & ailments that could permeate society on all fronts because the possible mutations could spread to all the genders.
We actually have a better chance at creating suitable mechanical life forms to do our work properly than creating a biological lifeform properly and then with biological forms we end up looking at Rossum's Universal Robots they took out all the unecessary parts of humanity to build the robots for slaves are we to trudge down that path again?