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. =)