"weight of a removable battery"

Why would a removable battery be more heavy? More bulky, probably and with more plastic also a bit more heavy. But not much, as the heavy part is not the plastic.

Maybe we could build laptop cases out of carbon fiber and get the best of both worlds? Or at least make laptop cases easier to open for servicing, with standard screws and no plastic clamps.

I literally just don't believe that this is a real problem. Old school laptop batteries were large, but how much of that is really needed? In theory you could add a connector and be 90% of the way there. Put a panel on attached with a simple screw and you're golden.