> Just because you can breed fragile animals for narrow purposes does not mean you can breed a better human.
Note that your whole argument hinges purely on the meaning of the word "better". Once you rephrase the issue in terms of "can we bread a human that's more like X or less like Y", the answer is "obviously, yes". Your whole argument is based on some subjective judgement as to whether these are "better", but it is completely irrelevant insofar as answering the question whether artificial selection in humans is possible, and whether it could significantly affect trait distributions -- the answers to these questions are yes, and yes.
By the way, we are already breeding humans more fragile, precisely due to increase in medical state of art. If medicine technology helps significant number of humans to survive and reproduce that otherwise wouldn't have, and it clearly and obviously does so, it will result in observable increase in traits that would significantly decrease the fitness in the context outside of human civilization, just the same as farm animals. I don't think that this complex issue of not just letting those people die without reproducing can be neatly wrapped up in a single word "better", but make no mistake: selection for more fragile humans is very much happening, right now.