If Musk’s X moves away from the name “Twitter” and the bird logo as completely as they seem likely to, those will become essentially undefended trademarks that someone else could start using.
Want to found Twitter all over again?
There are ways (legal tricks) to rebrand while defending the old brand. But these require detailed legal advice, disciplined ops, and a will to do so. I’d say that it looks like Musk is lacking all three of those at X right now.