This is her big mistake. Tywin was no fool and he knew that while the Lannisters were in debt to the Bank the Bank had a vested interest in their success. By paying the debt in full Cersei has allowed Tycho to wash his hands of the Lannisters altogether. After what we saw on the battlefield, we have a good idea whose position is strongest and who the Bank would like back. This clearly contributes to the dire situation in King's Landing with no grain and limited supplies. She will not abdicate, instead she'll force the dragons to destroy the city at which point the Kingslayer will be the Valonqar most assume he will be. The irony of this theory is in this case the Lannisters are undone by paying their debt."