But to clarify let me spell it out for you. Gone are the days where people want nothing more than to explore. There are always explorers like myself, but the majority of players are goal and rewards oriented. So if you release 6 zones all of max level content, its not going to be anywhere near as effective for Trion. If players don't have a goal to work towards then they won't do it.
Moreover if you release zones of all level 50 content they either make the zones incredibly easy and all the raiders will faceroll through them, or they make them require raid gear, in which case thats about the easiest way to kill the game right there. Unless Trion has a way to scale players up the content (not typically a good idea). Then it won't work. And if they do that what is the incentive then for goal/rewards oriented players to even do it. Like I said though I would like to think its eventually possible to make a expansion without increasing the level cap, I'm not so sure it is.