What would you say to someone who's made a poorly-written story and says, in response to criticism: "It’s only bad because you think it’s bad, I do not. What’s good or bad is only based on opinion, not facts."?

People don't realize that poorly written stories can be misunderstood or even ignored if someone gets tired of trying to figure them out.  That doesn't make them "bad", it just makes it difficult for someone to understand the story, the author's intent, or maybe even take them seriously.  That's certainly not going to be good for the author or their readers.  When we're trying to communicate solely via the written word .. It's really crucial to be as concise and grammatically correct as possible to get the best result.

True to a point.

But if it's bad due to incorrect grammar or spelling - that is fact, not opinion.  (Which is often the case of younger, self-published writers)