Fixing genetics bugs on a wider scale would be a medical game-changer. A good many debilitating diseases could be cured.
We can, for instance, add genes to plants to produce substances that bugs don't like and we can eliminate genes from animals that inhibit growth. It's not insurmountable to edit genes to "do more" or "do less"—if the answer is making more or making less of a protein we already know about. But changing the design of something is a hard challenge that we aren't good at. If increasing VO2 capacity was something that could have evolved as a beneficial mutation by way of increasing or decreasing the presence of a protein, it probably would have happened. But the fact that it hasn't either means increased VO2 capacity isn't great (eliminating muscle growth inhibition, as it turns out, isn't great) or requires more than just tweaking a protein amount.
Which is to say, gene editing is the ability to change the code. It requires separate capabilities to know what to change it to. This news is great for areas of research like cancer treatment (imagine taking a puff from an inhaler to cure your cancer, which is just ~2 gene mutations!) but much less exciting for "enhancements".
VO2 max capacity is an adaptation that all athletes develop through cardio training, so much so that there are drugs that make this adaptation in red blood cells abnormally abundant, like Epoetin Alfa/erythropoietin. If a drug can do it, making it permanent could be possible too.
As far as we know, the drug that Lance took makes you have high blood pressure, introduces iron deficiencies and (in people with previous heart conditions) may cause cardiac insufficiency, so it's not pure benefits.
In short, your argument of 'all mutations have benefits and drawbacks' is right, but if the benefits are that high, there's no question of who'll do it but what counter-indications are you willing to put up with for the massive benefits it brings in sports (and life!) performance. Lance said he felt like a god when he was on it, he slept like 3 hours a night, rode for 12 hours a day, and won the Tour de France with it, with no sequelae, neither (according to him, so take it with a grain of salt).
Before anyone says 'just take steroids, man', I'm all in favor for a separate league of juicers in all sorts of sports, be it team sports, combat sports and solo endeavors. To me it'd be super interesting to see what humans are really capable of.