Personal I think what he did wrong was changing the entire company system and rules .in fact he just changed everything so many restrictions and I guess that's why majority of advertising firms stopped advertising there. He just fucked up thinking everything could go his way. He should have followed the system a little and make changes more slowly. Even tho he is a genius ill say its not a very smart move from him 'Mr Twitter destroyer' 
I wouldn't go as far as saying that he changed everything in its entirety as there is still the main purpose of Twitter (now called X). There are just some changes that he made that are questionable in the perception or users and businessmen. However, X is still far from falling as many artists, actors and actresses, public figures, and brands are using it to promote and connect with their community and target audience. However, it is safe to say that Elon did some changes that made users unhappy for sure.
Indeed, the fact that Elon bought Twitter you would already expect a lot of changes since Twitter has a new owner. Elon Musk might changed almost everything at Twitter, there's nothing you can't do even the fact that most of the users doesn't like the new logos and features. I have seen a lot of artist that keep complaining that they don't feel the vibes at Twitter at all unlike before. But trust me, in just few weeks or months they will just get used to it cause that app helped them to provide their needs. From their reactions and what they've felt from the changes are really relatable and pretty normal. Imagine you were just sitting in your favorite spot in the park then it just suddenly changed with the design after all those years.