You seem to think 1k rooms is a lot.
It is not really. There are several areas where 1k rooms could be added and though you might notice the change, it really would not make much of a dent in those areas. Silly amount of work...Meh, not really, I could do that in 2 weeks working only 2 hours a day in any old diku. True, it is unlikely I would, because I am lazy, more likely I would work 1-2 hours, take my time and do 5-10 rooms a stint.
But that is all beside the point. The number does not matter, 100 rooms to this area, 30 to that, 300 to the other, flesh them out, or at least make them seem finished.
Adding totally new areas, now yes, that can be a reasonable amount of work, because you can easily get to several thousand rooms to finish a single "zone".
As to payoff...shrug, maybe exactly what you say, though some of us don't map. Or maybe a labor of love to have areas finished that right now only exist in room descriptions of things you can see far away. More places for outlaws to hide, more places to hunt outlaws, more things for the Byn, salarr or insert name of clan here to do.. The payoff is the world...with your way of thinking, hell, the entire game could be in one room and we could call it a mush...after all, what is the payoff to have anything coded, we could just emote everything.
But don't worry Reiloth, with the rate places have been closed, removed or shrunk down...I doubt you have anything to worry about in this matter.