I think what Leah means is not whether it's physically possible to return to a country, which clearly it is presuming there isn't a physical or political situation preventing it, but whether the place you leave is ever really the place you return to. As someone who has been in the USA now for 21 years I realize that while I could choose to live in the UK, and semantically speaking that would be a return to my country of origin, I would not be "returning home" as too much has changed in the country I left for it to be the same place; and in the process of becoming fully vested in the country I'm living in I have to let go of some of the roots - or find myself in an endless and tedious mental game of comparison while never fully engaging in the place that I'm living.
For me, the UK is a wonderful place to visit with family and friends around many corners, and I enjoy being a tourist in the country I grew up in. The pull to move is considerable but even if there weren't family ties holding me here I think I would still have to think very carefully before doing so. But I am sure that this is not the case for everyone, for example Rebecca's friends (previous post) who came to the US to achieve certain objectives with the end game always being to return home. I imagine they were likely able to return with little difficulty as in their minds they never fully left.