"Adding a big lobby button at the top will make it faster to identify and hit the lobby button, while persisting the existing lobby button on the bottom."
The clear solution is to make the existing lobby button twice as wide, right? I'd have been very interested to see the results of a layout like that.
That said, elevator buttons are in need of redesign. I have never in my life hit the "hold open" or "close now" buttons because something about the triangles confuses me about which is which, in the short time I have during which they'd take effect.