before start, sorry for my english, i'm french and come here. we have the same topic open on our forum.
a lot of us have the same concern about the generation of gil at level 50.

some point to add to the op statement.

most of the monster 45+ humanoid don't drop anything, exept the small drop rate for the imperial monster the other one don't drop stuff. (i did test out in natalan for 1 hours 0 drop, item or gil)
for a battle class it's impossible to generate gil (and we don't talk about sell/buy stuff on AH). like the op have said every action a battle class can do at 50 will only lead to a net loss of gil.

indeed we can try to get gil from gathering and crafting class. but not everybody love do this... and if the game force a player to do something for be able to enjoy the game... it's a huge error of game design.