If you can live a sedentary lifestyle and still produce children, then evolution doesn't care. In fact, an indoors lifestyle is much more fit in the longterm than a life of hard labor.
Evolution isn't trying to create the Platonic Ideal of a human. It just is a thoughtless mechanism to efficiently reproduce DNA.
Hunter/gatherer humanity did not have the luxury of a sedentary lifestyle, so it did have a negative impact on your ability to produce children for a long time.
> In fact, an indoors lifestyle is much more fit in the longterm than a life of hard labor.
I think "hard labor" is a bit extreme, but even so, working a physically demanding job is still better for your overall health than a sedentary lifestyle of office job + sitting on the couch watching Netflix.
Again, evolution does not care. As long as you can survive to about 40 (long enough to raise children), it makes no difference.
Or maybe people will just learn to exercise regularly and stay healthy? That seems more likely than the collapse of society.
Maybe you should be asking the opposite question like why should evolution reward sedentary life? Human history involves daily exercise in some form that is more strenuous than a long walk and that's clearly connected to our general health and well-being both mentally and physically (why should these be separate to evolution?). What we're doing today is the exception, we shouldn't expect everything to work out as before.
I imagine we're in a temporary lull. Either modern society will collapse and we'll go back to farming or hunting and gathering, or we'll invent a exercise pill. There is effort in this direction:
http://qz.com/516757/scientists-are-working-on-an-exercise-p...