In some regards, it could be said that Sims 3 was the game that overshot its goal... Just taking the amount of content they released for sims 3 itself, none of the mods, just the content and store stuff; they basically broke their own game by putting so much in that the game engine couldn't handle it... I'm using no mods with Sims 3 and my game is lagging persistently and consistently...
I think that's why they've been so cautious about adding new trait slots or new traits for Sims 4. Even if we may not like the approach theyre taking, when looking back, its understandable why they take such a route... With traits, worlds/maps, styles, etc. We may not like the approach, but we can understand why it was done.