That's giving them a lot of credit. I still think it's simply due to limitations in their booking system/IT. Same as the problem between GMT and AEST for registration to the offer or not being able to display the 2 different seat maps QF11 to JFK and not being able to change a booking online if 2 change of schedules have occured since ticketing, etc...
It's easy to associate some business benefit in not fixing the voucher loophole, but QF have so many IT limitations that I don't think they can all be justified by some secret master plan to increased cash inflows. As I mentioned before, it took them a few years to fix the non-upgradable fare loophole, at the time one could have said QF was happy with the loophole because people were spending their points to upgrade, so less FF points liability. But in the end they fixed it.
Anyway, just my 2 cents..