Then your desktop becomes more portable, collaborative, hackable, and open. Windows or OSX could make their local scripting environments more open and universal

Then your desktop becomes more portable, collaborative, hackable, and open. Windows or OSX could make their local scripting environments more open and universal but they have 0 incentive. So eventually the browser will do it. Inefficiencies will be squeezed out over time. Tabs aren't really windows although they will be (or vice versa).