Destiny: The Taken King includes all Previous Expansions regardless what version you buy
Bungie announced that now all versions of The Taken King, the latest expansion of Destiny, include The Dark Below and House of Wolves will include it.
As you remember, if you wanted to purchase the full experience of Destiny it was necessary to acquire 3 expansions.Considering that The King Taken costs $40 USD and The Dark Below and House of Wolves are priced at $20 each, some players had to spend more than $80 USD for all content.
It is true that since the launch of The Taken King, Bungie offers The Legendary Edition, which includes all existing expansions for Destiny and the base game. But now you need not buy that edition to enjoy the full experience.
In December 2015 Bungie said that would change the business model of Destiny. According to studio members, Destiny will no longer receive more downloadable content for payment but free events .