There are four main characteristics that can be seen in fans: investment, discrimination, productivity, and community.
1. A fan has an emotional attachment to the object of adoration. He may spend significantly more money, time, and energy on this object, for which he often feels a sense of "ownership" that separates a fan from a regular consumer. He, for example, would react strongly if someone did something they deemed to be wrong, e.g., cutting funding for his favorite franchise.
2. Fans differentiate strongly between objects of which they are fans and objects of which they are not. This also serves to build a community, establishing a boundary between fans and the "rest of the world." Discrimination also exists among fans in the form of "favorite actors, games, characters, shows, etc.," an accumulation of canon knowledge (elitism), opinions, and interpretations.
3. Fans take pleasure in manipulating and interpreting the meaning of objects they consume. Although the original ideas of producers may differ, this is not because of faultiness but rather for the appropriation of it. Gossips, discussions, and derivative works—fanart, fan-games, and fanfiction—are some great examples of fan productivity.
4. Fans want to talk to and share their adoration with like-minded individuals to heighten the pleasure.
Why am I telling you this? The fact that fanworks play an important part in keeping a fandom alive is a good reason for me to disagree with you. Virtually any discussion can and will poison your interpretation of the original work, but that doesn't mean we should stop doing it altogether. No, that is just silly.
Recently, I was reading about the early days of the yuri before it became an established genre. Suffice to say, derivative works played a huge role in establishing what we know to be yuri, with one survey claiming that more people read and have been introduced to the genre through derivative work than its original counterpart; the doujin artists themselves becoming yuri mangaka afterwards. Notably, Yuri Hime employed a similar strategy of fan interpretation to engage mainstream consumers and ensure the genre's continuity through the anime adaptation of 'Yuru Yuri.'
Then there is fan-driven community like Touhou. The official content for the franchise is extremely small compared to what the community has to offer, and that is the beauty of it; there is a reason why such niche games has such a longstanding and strong fan base.
People profiting of other's work is a valid concern, and I share that too. But I personally don't think it is part of the conversation here. On the note, calling fanfictions to be creatively bankrupt is a weird take. Because, going by what I've explained so far, fanfictions are not a show of creative expression but of a fan's pleasure in twisting the original meaning to his liking.