They are renaming Twitter because, I think, they want to expand to services unrelated to tweeting and messaging.  But then, plenty of companies have changed businesses without changing their names.  The cable channel AMC (American Movie Classics) used to show only old movies, but they have since changed their business completely to show shows like Mad Men and Breaking Bad.  And they still retain the name AMC.  The MTV channel is another example.  Like you said, sometimes the name-brand recognition is worth a lot.  But then, we aren't billionaires like Elon Musk, and our definition of "worth" may be different from his.  If you see a penny or nickel on the street, you may not want to pick it up because it just isn't worth it.  So what is highest amount of money lying on the street for Elon Musk to NOT think it's worth it to pick up?  It's about $30k, if you compare his wealth proportionally to ours.  He would say, "Oh, that's $30k lying on the street.  Naaah, not worth it."  Likewise, he may think that Twitter's name isn't worth that much to him.