I thought so too, in theory it shouldn't matter because the sims4 just loads one neighborhood at a time anyway. It doesn't matter in which world it is. But I guess it's the total amount of worlds and lots that matter.
As far as I understand it's not possible because they want to support lower-end computers and they also want to continue the sims4 for many more years, too many large worlds could create a sims3 situation where we have to start disabling packs in order for the game to run well. Too many sims and lots could make your save file so large that you will start to have huge loading times and bad performance.
I hope it doesn't mean that worlds are going to get smaller and smaller and performance worse and worse the longer they continue this game.
