Actually, the jury is not yet in on long-term (many years) exposure to microgravity. This is one of the major things that needs studying in places like the ISS--raising animals over several generations and such.

Many people think that overly long exposure to microgravity is detrimental to health in many areas, and some studies support this.

Maybe mammals can adapt to microgravity permanently, but be unable to return to normal gravity after a long period. This is just speculation on my part.

Maybe with enough study, medical methods can be found to allow humans to live in microgravity for many years, but still be able to be readapted to Earth. This is also speculation.

The easy way out is to design very large space colonies with 1-G rotation, but if any way can be found to adapt to microgravity or low gravity, colonization of solar space would be an order of magnitude easier. A zero-G asteroid colony is real easy to build, and one could live on any of the smaller moons or planets.