Your theory about pack related content not being updated for other packs is just wrong, and we've seen it time and time again that new systems added are regularly updates.
The Parenthood traits are constantly updated, such as doing laundry raising responsibility, taking care of pets raising responsibility.
Clubs constantly get new activities every pack.
University adds new careers every pack.
The sims 2 added several personality improvements tied to expansion packs, but the sims 2 sims had solid personalities on their own.
That being said, it should absolutely be base game, but I am fine with it being like lot traits where the system itself is base game but new things are pack specific.
