Well none of the rumors I've heard have stated it's solely for one misfire. It's for the well publicized production issues on multiple films, staff not getting along, and for the misfire, as well as some other stuff. There's been word that others have been approached and Kennedy may step down in September. Of course this is all from the rumor mill, so who knows how true any of it is, but if true it's not solely because of Solo's failure. Disney doesn't like bad publicity, and the Star Wars brand as a whole has had some very bad publicity the past year alone. Including some that's lead to new laws in some places (Battlefront 2).