This is fantastic news. And I am supremely skeptical that anyone claiming to not want to buy the game now because of this, was never going to buy it in the first place/are just trolls.
Just about every franchise has plans for further games. And any given entry in a series still has a complete story, especially first entries.
Think about the original Star Wars -- it still told a completely contained story, and only later expanded out. Just about all trilogies work like this. The one major exception is Lord of the Rings, but that was originally one big book, so that is why it is the way it is.