One possible reason could be a lack of time. Maybe Ash has become too busy with other aspects of life, like work or studies, and can't find the time to write fanfiction anymore.
Maybe the loss of a very dear Pokémon friend. If something bad happened to a Pokémon he was extremely close to, like it getting seriously injured or lost forever, it could be so devastating that he loses the will to continue his journey and thus gives up.
It could be that in this fanfiction, Ash had some personal issues come up. For example, he might have received some bad news from his family back home that made him lose focus on training and eventually give it up. Another reason could be that he found a new passion or interest that diverted his attention away from training.
It might be that Ash has other priorities now. For example, real - life commitments such as work or study have taken up too much time, leaving no room for writing fanfic.
After Ash gives up on Charizard, the story may shift towards the development of the Pokémon that were previously overshadowed by Charizard. The fanfiction might show how these Pokémon grow and become stronger in Charizard's absence. At the same time, the fate of Charizard itself could be a major plot point. It could be on its own journey, facing new challenges, and potentially evolving further without Ash. This could set up a future reunion between them that is full of drama and excitement.
His Pokémon would be very confused and sad. They are used to following Ash on his adventures and having him as their trainer. If he gives up, they might lose their purpose and direction.
The fanfic community is quite large and diverse. While Ash giving up might be noticed, the community as a whole has a lot of resilience. There are always new stories being created by different people. But it could also serve as a reminder to the community to support its writers better, so that others don't feel the need to give up like Ash did.
The story might take a more somber turn. Snape could become more withdrawn and focus solely on his own pursuits. For instance, he may throw himself into his potions research even more intensely.