Actually that's not true. We currently don't know the bottom of osteoporosis, all we observed was a gradual decline in bone quality directly proportional to the length of stay, some astronauts returning with bones of 80-year olds, in a few cases irreversibly so. All other declines like muscular mass, amount of circulating blood, heart shrinkage have some equilibrium beyond which no adverse trend continues. With osteoporosis we never observed this, so going to long flights, we might lose all our bones. Supplements, training etc. help a bit, yet the decline + increased Ca excretion persists despite. Not mentioning this is not friendly to kidneys as well and nobody wants to end up with stones developed during a spaceflight.