RE: Alone and sorrounded
Gotta say, I agree with your initial premise. Humanity will certainly devolve into a chair-bound species of interactive goons... but not for long. Though exercise machines would help, eventually people would simply start to wither as their bodies broke down from a thousand illnesses (infections, vascular collapse, etc) each more tenacious than the last. Simply because humans just aren't evolved for such, and never will be.
It's like microgravity. Folks think the most dangerous part of space travel is lack of food or oxygen... but it's not. Though those are tremendous primary concerns, even if supplies of both were 100% stable, people don't do well. It's not muscle atrophy either, everyone has heard of that. It's that Every. Single. Cell. has been developed specifically in (and for) a Gravity 1.0 system. Going outside of that for long, even to something as mild as a Grav 0.95 environment would likely be disastrous, longterm.
So I don't think the virtual reality scourge, as I put it, will carry on so far as you think.. Probably a good generation or two (including the millennials) will be lost to it, but if civilized folks ain't got their shit together after then, there's a whole rest-of-the-planet waiting to step into their place in line. =)
I prefer to think that the people that will live on Mars for example, or travelling to other stars, will be able to adapt and evolve differently. Of course it is not easy. But it will be possible. They won't be able to go back to Earth and live with that strong gravity. Many will die, but some of them will evolve. Look at all the diversity of animals on the planet constantly evolving. Humans are no different. It's like telling a minnow... you are never going to be able to live outside the sea, in that solid rocks out there... don't go there you will die !!! You will never be able to breath out there or have legs to walk. One thing is sure... a minnow cannot become a whale in just one day hehe Give it time... a lot of time ;-)