Yes, there are likely fanfictions out there with such a plot. Fanfic writers often explore different and sometimes controversial storylines. However, this goes against the strong bond between Sasuke and Naruto in the original series.
Naruto would probably be deeply hurt. He values his relationship with Sasuke so much that such a betrayal would shatter him. He might become withdrawn or angry.
I'm not sure if there is such a specific fanfic. Fanfic topics can be extremely diverse, but this particular scenario seems rather untrue to the original Naruto storyline and the relationships established in it. However, in the vast world of fanfiction, almost any idea can be explored, so it's possible that someone might have written it.
Fans would likely be divided. Some fans who like exploring unconventional and darker storylines might be interested in such a fanfic. They might see it as a creative exploration of the characters. However, many fans who are loyal to the original relationships in the Naruto series would probably be outraged. They would feel that it goes against the essence of the characters and the positive relationships that were built up over the course of the series.
In fanfic, if Naruto is depicted as cheating, it can really change the perception of his character. It might make fans who love the original Naruto's loyalty and integrity feel disappointed. It could also be a very controversial plot point that might cause a divide among the fanfic reading community.
If Naruto gets cheated on by Sasuke in a fanfiction, it could greatly disrupt the traditional bond between them. It might lead to Naruto feeling betrayed and could potentially make him more cynical. This could change the direction of the story, perhaps leading Naruto to focus more on his own self - growth rather than their relationship.
Yes, there might be such fanfics out there. In the Naruto fandom, with a large number of fans creating various types of stories, it's possible that some would write a story where Naruto destroys Sasuke in a way that could be seen as Sasuke - bashing. However, these kinds of fanfics may not be in line with the overall spirit of the original work which focuses on their complex relationship and growth.
I'm not entirely sure if it's a 'popular' trope. Some fans might be interested in exploring such dramatic and controversial storylines in fanfiction for the shock value or to explore different character dynamics. However, many fans of the original Naruto series who ship Sasuke and Sakura would likely be disappointed or even offended by such a concept.
I'm not sure if there is such a specific fanfic. Fanfiction can cover a wide range of scenarios, but cheating themes might not be very common in the Naruto fanfic world, especially considering the strong bond between Sasuke and Naruto. However, with the vast amount of fanfiction out there, it's possible that one exists.