This makes perfect sense... You just aren't comprehending it well... There will always be things to go, just not all @ the cap... Once "Casuals" do all the content, they usually are gone. So the reason to always keeping new content every 6 months it to keep the game fresh. Once you are "75" or "max rank" for 1+ years you learn your job, you are just repeating the same thing constantly, rarely does battle change other than maybe required classes to make that battle easier. With new rank, new content etc.. you get new abilities, new enemies, everything changes, and possibly what was good @75 is no longer good @80, which makes you adjust your game play and grow with the game. They can "Cap" these places as they grow so you cant go in higher than the Content allows, which keeps there 20 places to do, but always having to learn and strive for the next goal is what keeps people playing.
Expansions are what should increase the rank caps.. every 6 months new expansion, new rank cap, new abilities and possibly new jobs/classes.