There are three active possibilities for Go Set a Watchman. In order of likelihood:
1. Everyone forgets about it in a few years. By all accounts, it is not a good novel, and what makes it notable is only the fact that it differs in tone and characterization from To Kill a Mockingbird. The book is properly considered either as juvenilia or as something close to a last novel/posthumous novel/unfinished novel. If the former, people will recognize that it is not particularly interesting on its own: Trimalchio to The Great Gatsby, or "The Waste Land" before Ezra Pound edited it. If the latter, it will go into the same camp as The Last Tycoon or Hemingway's last novels: for completists only. I mean, Jane Austen has juvenilia and posthumous fiction, and no one pretends like either is part of her mature work.
2. It kills off To Kill a Mockingbird in high school curricula, and thus kills the novel altogether. The complications to "Atticus" as a character deprive him of his moral force in Mockingbird, and it's an awkward thing to try and explain to cynical teenagers. The novel can't serve as a simple (not to say simplistic) morality tale, and it becomes a historical curio of a particular time and approach to racial issues, a mid-20th century Uncle Tom's Cabin, to speak of once-influential works on race in America that are no longer widely read.
[A wide distance of probability]
3. The attractions of a novel wherein the narrator is a thinly disguised version of the author who moves to New York City and discovers that even the nice-seeming southerners are terrible racists and her father's not the hero she once thought he was, is a perfectly fine sentiment of the moment, no matter how cliché it is in all details. It feels more real, which is to say dirtier and more degrading of the human spirit, and thus will be taken for the 'adult' version of the children's story. Assuming she is of sound mind (a big assumption, where all people with opinions have vested interests in those opinions being true), then it seems to indicate that Lee didn't understand the value, importance and meaning of her own book. This would hardly be the first time such a thing were true, but it'd be sad all the same.