PA WILL NOT LONGER BE SOLD?

There are plenty of games that do this. I've got 4 versions of Dawn of War in my steam library, 2 of Dawn of War 2, 2 of Sins of a Solar Empire, 4 of Civilization 4. There's nothing wrong with making a stand alone expansion. In fact I prefer them, it means newcomers don't have to buy the original game and the expansion, they can get away with just getting the expansion.