The original TLOU will not change because there is a sequel. And if a new story bothers you so much you can always not buy it/play it/care for it, simply ignore it and you will never know. But do you have the integrity to do that?
The mere existance of a sequel ruins the integrity of TLOU. That was not a story that needed one. "Don't play it" is not a pacifier to that. If someone writes a sequel to 12 Angry Men, I'd obviously skip the thing, but that doesn't mean that there isn't a problem with that. It just means that I'm not consuming the problem. Creating a sequel to the TLOU completely shits on the integrity of the story that game tried to tell.
TLoU was an amazing game made by an amazing team of developers who have one of the best track records for videogames in the industry. They are an experienced bunch of individuals who have well structured projects that always make sense to them and their standards. Integrity is worth NOTHING to an industry and consumers who don't care for it. Are you honestly expecting TLoU 2 to be this game that ruins the first one? Do you realize how many movies, books, videogames and stories were thought 'complete', or could exist in their own right, before sequels were made to great success in the eyes of producers and consumers alike?
How do you know that ND has nothing else to visit in the world of TLoU universe? lol, how do you know this? What does the setting have to do with the integrity of TLoU? Did you write the laws of the universe that dictate "If someone tries to write a sequel to a game, it will lose all intergrity!!" That's like saying The Lord of the Rings books/movies shouldn't exist since they expand upon the Hobbit since the Hobbit is its own complete story.
TLoU was great not because of the plot or the settings or the circumstances that unfolded but by the progression at which the story was told, character development and many other things. Setting/plot have nothing to do with quality of the first game/book/whatever or even its sequel as evidenced by the fact that there are critically acclaimed content from every imaginable setting/plot possible. Any plot within any setting can be great and complimentary when done correctly and this applies to sequels as well.
TLoU sequel may not have the same plot in the first place, it may just take place in the same universe but be a whole new adventure a la The Hobbit and The Lord of the Rings. Is The Lord of the Rings a shit in the face to the Hobbit? Is The Bourne Supremacy/Ultimatum a slap in the face to the people who like the novel or movie of The Bourne Identity? These, and many, many, MANY stories can exist by themselves or were considered complete before sequels were made.
The Bourne series being a good example against your claim because it is very linear but was able to expand greatly not because of setting or plot but because of the development of the story and the characters. TLoU team is a master at this craft and the sequel should not dissapoint in the expansion of the huge potential that this universe has.
Bottom line is, there's always a story to tell and if you think TLoU sequel quality is endangering the integrity of TLoU then thats your sorry problem. TRoU will sit back and enjoy what may be the game of that year.