X might be a fine (if not a little cheesy or adolescent) branding decision if you're starting from scratch, but we're talking about a company that literally owns the verb. Companies would kill to be in the position to have their company name be established as the primary verb in their industry -- even other social networks like Bluesky use some variation of "tweet" to indicate a post -- and throwing that away in favor of a generic letter would be akin to Kleenex giving up their brand name to be known as K.