Judging by the coffee shop I'm in at the moment where about 1/3 of people have their machines plugged in, I don't think plugs are dead yet (not to mention at home or in offices, of course). Part of the problem is that the battery life is not yet at the level of phones. My 11" 2015 Macbook Air advertises a battery life of "up to" 9 hours web-browsing or 10 watching videos, but in practice I've found it closer to 6-7 while working actively, if I've got several apps open, wifi on, and screen brightness up. Maybe switching from Firefox to Safari would help, dunno (but I'm pretty tied to Firefox at the moment). I can get 10+ hrs on planes, but there I have the screen brightness turned down, wifi off, and am mostly working in a terminal vim.