I believe this should be incorporated into the original software and not an expansion pack.
Not all users would interested in it, and developing add-ons professionally costs a lot of money. The base GSX was released in early 2012, and updates to its core program are still coming and are STILL FREE, after 7+ years. We could have made a couple of small sized airports and sell them, in the time it took to do the free PBR update...
But there's no way we could keep adding stuff over and over, forever. Doing expansions which are obviously *optional*, means users can decide for themselves to continue supporting the product for updates, which will keep coming for the Base product THANKS to the sales of the Expansions, without feeling compelled to buy something they might not be interested into.