“Musk wants to go to Mars. O’Neill wanted to build an orbiting space cylinder. Why not do both?... An O’Neill cylinder (also called an O’Neill colony, or Island Three) is a space settlement concept proposed by American physicist Gerard K. O’Neill... [that] would consist of two counter-rotating cylinders... [and] provide artificial gravity.”
I like the idea of pursuing both, but both will have enormous challenges. And permanent off-earth colonies will probably require new energy sources and forms of propulsion.
The artificial gravity created by rotating a ship or other manmade dwelling is not the same as gravity because the acceleration is not linear which causes the Coriolis effect. It is impossible to say how this may affect living things in the long run. People, animals, and plants may not be able to adapt to it long-term any more than the rough conditions of Mars.
Your remarks are cogent. But that is the only way to get off planet. I agree with Musk and Heinlein. The Earth is fragile and unless we learn how to get to and live elsewhere, we are doomed. It won’t be easy, but anything worth having never is.