We will get at least one more patch in Legion ... hell, they could even give a Ruby Sanctum type raid before it is over. The fact they didn't announce a new expansion now does not mean we will have 16 months of Argus and given some of the Argus content ... it isn't just Argus.
Legion isn't how I would have wanted it. I would have had invasion in older content ... Make leveling outside during the expansion fun (if they wouldn't do a full world revamp again. I think Cata failed there because they didn't really know what to do. I would have had unlockable Race/Class combos ... like finish the the Order Hall quest for Paladin and unlock Night Elf Paladin as playable (of course this screws over Hunter and Warrior ... and Mage and Priest pretty much as the races that aren't those don't make too much sense to become them ... but there could other rewards there).