I think Amtrak Virginia would take in a orphaned Cardinal day train stub as far as White Sulphur Springs, just over the WV border. Whether WV would pay to extend that train to Charleston, I would estimate as a coinflip; Huntington is unlikely on the merits but internal WV politics may intervene.
IN was eventually strong-armed into supporting the Hoosier State, but might be more willing to support a daily train to CIN (whether or not OH can be persuaded to assist) if the schedule was adjusted to fit the needs of IND and CIN, which the present Cardinal doesn't even pretend to. How late you can arrive CHI westbound/depart CHI eastbound, i.e. how many connections you're willing to break, is something of an open question. Thruway bus departures from IND already connect to the CONO, Eagle, Chief, and Zephyr on strictly superior schedules than are available from riding the Cardinal/Hoosier State, so there might not be much of a hit to ridership by emphasizing them as the connection to the west while the train is the connection to Chicago. You'd just have to extend those buses to/from CIN. (Eastern riders changing at CHI would presumably be consolidated to the Cap and LSL.) I'm not fully convinced that that is an optimal solution (there are many, including several members here, who will simply refuse to ride a bus, no matter how nice it is or how much time it saves), but I'm no longer quite as nervous about it as I was previously.