Few things change the world overnight. That doesn't mean it isn't a bad thing.
If it doesn't work out, it will likely become another vitriolic cesspool of a website enabling an echo-chamber of like-minded people (some bitter, some zealously idealistic). This tends to happen when a website cannot gather the population necessary to maintain a diversity of ideologies. Failure, in this case, is the best case scenario.
If it does work, it provides Elon Musk (and his successors, who would be functionally appointed by investors) with the infrastructure necessary to damage critics and push a narrative. Especially if the topic is sensitive (like government corruption, or corporate corruption, or any scandal) as those situations tend to rely on anonymous testimony as many people do not want to lose their jobs or their lives. Anonymous testimony is already often maligned for being impossible to verify (by necessity, mind you) and would make any substantial scandal an easy target for someone like Musk to sick a mob on (for even just fudge the numbers to make the website or whatever look bad as most people just look at the numbers). A powerful propaganda tool.
Besides, there is already enough in place already to prevent outright lying by the media. They are called anti-defamation laws, most countries have them. The main issue there is that not everyone can afford a lawyer (hence why the news is more careful reporting on well-to-do people and corporations). If Musk wants an actual practical way to contribute to a more honest media, perhaps he could start a firm specializing in defamation cases. But that wouldn't be flashy or really contribute to that cult of personality he has going on.
