Except that almost all AA departure lounges also operate as arrivals lounges and do allow access with the arriving boarding pass (presuming you are otherwise allowed access to the lounge as a departure lounge), and most Admiral Clubs do have showers -- thus the backward and much maligned AA lounge system actually has a significant advantage over QF in this case. And it doesn't even take capital investment, just a change in the rules (or putting up an "arrivals lounge" sign). Obviously QF doesn't want to do this, for whatever reason.