Exactly, from radiation to muscle loss - humans are simply not meant to live in space. BUT - it doesn't mean we won't eventually, especially if forced off Earth due to some human made or natural catastrophy. Humans may need to evolve for space living though. It may take many generations of evolution and adaptation and natural selection.
Maybe I will vacation back to lowly earth once in a while :) . But seriously, astronauts do up to a year fine right? Has anyone gotten seriously ill or died? I've heard of bone density loss, but do you really need dense bones if you will not come back to earth?
The ISS is in a low enough orbit that it is protected by the earths magnetosphere.
Radiation, space anemia, immune system issues, and probably a lot of other things we would know more about if people stayed in space longer.
Your body is built to expect gravity to pull blood down. The circulatory system thus pushes blood up. When in space this creates problems.