Sounds like blasphemy, but what could KC get via trade for Charles?
JC will be entering his 8th season - big $ and maybe you can get something to help rebuild. You have Davis and you can always go get another RB in the draft or find a cheaper alternative in FA.
I don't see anything in FA at QB that is really better than Smith, unless you like Vick, Sanchez, Hoyer ... don't know that I'd trade for RGII quite honestly.
Pick up some draft picks if possible, start to fix the OL - and beef up the passing game. Is KC going to be able to afford Houston, retain JC and fix the holes?
Go all in and realize that Charles is at that point where you may be better off getting something that can help you for the next 4-6 years.
http://www.spotrac.com/nfl/kansas-city-chiefs/cap/2015/