Well at least they are sacking them now BEFORE they start filming.
I'm guessing Rian would be the first pick , the lack of trouble surrounding his production of VIII makes him the obvious choice. Plus its been his script and direction that seems to have given the characters the development they needed after TFA,and,reading between the lines of whats being said, enough creative thought put into it that would perhaps bring the new trilogy to a more satisfying and conclusion if he were given the chance.
But again, whatever they are going to do, they are going to have to do it fast. When Jack Thorne was brought in it sounded to me as if it was the story that they are really stuck on, and with CT leaving now that confirms it. Shooting is due to start January 2018, so that doesn't leave alot of wriggle time to sort it all out and make a good job of it. Rian is the obvious choice to manage it to the end, because its one they REALLY have to get right for the future of the franchise beyond IX.
From the studio side of things I just think that sometimes it takes a while to find the right people to produce the kind of film they are looking for. There is no doubt in my mind that Lucasfilm saw the success at of the Marvel cinematic universe and tried to let it take itself in that direction, but the truth is the Star Wars universe doesn't allow either the stories or characters to be played with in the same manner. As the Wook says its got a certain feel to it.Getting that right is proving trickier than they thought.
Hence the mess with "Rogue One" , originally shot as a supposedly dark and gritty war drama like a "Private Ryan" in space for older audiences, then reshot to bring it into line tonally with the OT ratings. Then more or less the same problem with Solo this time the stronger comedy angle getting hauled back.
If Lucasfilm could find somebody to nail the kind of game changing tone they need to the main films as Marvel did with "The WInter Soldier", with a stable writing team like Chris Markus and Steve McFeely and somebody like the Russo brothers to shoot it then everybody would be laughing.
But the truth is at the moment they are having to pullback on the reins harder than they anticipated because its fairly obvious the ones they hoped would deliver what they were looking for can't. And unlike some other major studios they are actively reacting to it now. With Rian there doesn't seem to have been any drama at all. So I really hope they go with him.