I'm the author of which-key. For what it's worth, I have no doubt that the package could be improved. I do have one idea off the top of my head for how it could be more tightly integrated with emacs.
As far as the amount of output is concerned, it is possible to filter out keys and customize the output to some degree. You can also limit the size of the window and control sorting. Of course, emacs does actually have that many bindings, and that may not be apparent to people unless they are written out. The original idea was that there would be a significant delay, so that you wouldn't see the window until you were "stuck".
In any event, I don't have a strong opinion on whether this should happen, but I have time here and there to help.