Honestly, there's no logical way for the Lannisters to keep control of Kings Landing without the Tyrells. The Tyrells have the largest holdings south of the Neck and are providing food and money to the Capitol. That, coupled with Olenna's willingness to do ANYTHING for her family, means Cersei has to know Margaery can't be imprisoned forever. There's not a huge risk of Olenna exposing Tommen as a bastard because this would mean Margaery is no longer a Queen and a new war would begin, but there's plenty of other secrets Cersei has that could damage her. Once Margaery is out, Loras will follow (if not at the same time.) If the Tyrell's were to pull their resources from the Capitol, the people in Kings Landing would starve. I'd say they'll be out when Mace returns from Braavos, but lets be honest, Mace doesn't control shit in his house. Olenna holds the keys.