Some would argue that nature gave us our limbs to use them, and as such voluntarily swapping them out would be something to frown upon. But the way I see it, there aren't any rules that say I can't or shouldn't. Besides if we're going off nature, nature also happened to give us humans incredible ambitions to achieve to new heights - we've never come to a place or point in our history and said "this is good enough." We always desire improvement. So what happens when our ordinary body parts don't cut it? Say I want to climb to the top of Mt. Everest or cross the Sahara Desert on foot, in one go. Human biology just isn't cut out for that, and yet we were still gifted with this insatiable need to achieve more and further. Why not change your body to allow it to actually be able to follow what your mind desires for it to do, when before it wasn't possible?
Another thing I hear a lot is that such modification would make you less human, but I think I lot of people don't actually understand what it means to be human. You'd be less animal, yes, but you don't usually hear people say that. Being human is, contrary to the context the term is commonly used in, being above and beyond being just an animal; being distinct. We've already long surpassed them in our cognitive abilities - how do you think we rose to the top? I'd even go as far as to say it'd make one more human, since unlike animals, if we really want to be able to do something, our body can now actually keep up with our mind in doing it. Never run out of breath, never suffer from fatigue, never be held back by outside pain, never be held back by any biological limitations you can think of. Being able to do that would make us even more distinct as human than before, and we have the technology now to maximize our chances of living to see this. It's time that we seize the full potential of the human mind and see just how far we can truly go~