Disclaimer- I'm not predicting that will happen, just answering your question.
Downside 1: Rondo continue to play like he played last year (also a contract year), get to the bench and destroys the already bad team chemistry, the defense of him and Belinelli together is horrific and the team doesn't even sniffs the playoffs, Cousins wants out and the Kings can't even tank because they have no control on their picks.
Downside 2: Rondo plays well, but not good enough to make the playoffs, he end the season by living and the Kings struggle to ink guys on a summer in which almost every NBA team has cap space due to the rising cap. Kings can't build through the draft because, again, they have no control on their draft picks.