The point around gravity is weak. We have exactly two datapoints for long-term human habitation: 0G and 1G. There is a vast spectrum between the two, the effe

The point around gravity is weak. We have exactly two datapoints for long-term human habitation: 0G and 1G. There is a vast spectrum between the two, the effects of which we simply do not know one way or the other; pretty presumptuous to assert any particular shape of that graph without even so much as a third data point. Can we get away with 0.5G? Can we get away with alternating periods of 0G and 1G? Lots of entirely-unanswered questions there. Hopefully, with new manned lunar missions on the horizon, we'll be able to collect more data on that front. In any case, both centrifugal gravity and oxygen production are readily-solvable problems. We don't strictly need the artificial ecosystems in question to fully encompass what's on Earth to be suitable for long-term habitation; they just need to be sufficiently reliable.