We'll see what happens to browser performance when most apps on the Win8 desktop are HTML/JS/CSS based and when typical displays have 300-400dpi... We'd probably get better performance if we resurrected NeWS.
300-400 dpi on desktops is a silly idea. 200 dpi is excessive for a display that sits a couple feet away from you. Even for laptops which have screens that are on average closer, 200 dpi is pretty much all you need. Going much beyond that gets you very little in terms of perceived quality while costing a lot more to manufacture.