Cersei Lannister is one of the most compelling and well-written characters in both Game of Thrones and the A Song of Ice and Fire novel series it's based on, but it’s hard to call her one of the best. That’s because such a word has positive connotations, and Cersei is a character defined by her negative actions. Kind-hearted moments from the newest Queen of the Seven Kingdoms are few and far between, but even rarer are the times in which Cersei displays any level of competency - she can most often be found making the kinds of decisions that will hurt both her and all of Westeros in the long run.
During Season 7 of Game of Thrones, Cersei pays off the Iron Throne’s debt to Tycho Nestoris and the Iron Bank of Braavos, and in doing so, she assumes once again that she’s made a wise choice. As several fans have pointed out online, however, this is a decision that will probably come back to haunt Cersei - and it might just cause her downfall in the series’ final season. Who needs the Night King to wipe out Westeros when Cersei Lannister is already doing a fine job?
Before examining the potential consequences of Cersei Lannister’s decision to pay off the Iron Bank in Season 7 of Game of Thrones, it’s important to put that decision in context. While it’s true the Iron Throne - as represented by Robert Baratheon and his successors - did owe the bank some money, that’s not the bulk of what Cersei was repaying. The crown had previously borrowed the majority of its funds from Tywin Lannister - who in turn was loaning it from the Iron Bank.
In the A Song of Ice and Fire books, the Lannisters derive their wealth from the gold mines underneath Casterly Rock. But in the television adaptation, it’s revealed the mines aren’t producing anymore and that Tywin has been borrowing from the Iron Bank to compensate. Thus, in paying off the debt in its entirety, Cersei is forgoing the money the Iron Throne owes her family and consolidating the supposed Lannister fortune with that of the crown - a risky maneuver, at the very least.
One of the ongoing themes of Game of Thrones revolves around the idea that “power resides where men believe it resides” - and there are many legitimate reasons to believe the Iron Bank of Braavos is the single most powerful entity in both Westeros and Essos. As the primary money-lender in the world of Ice and Fire, the organization bankrolls the majority of the world’s conflicts; after all, soldiers don’t fight for very long if they’re not getting paid.
The history and lore of A Song of Ice and Fire tell that the Iron Bank has toppled kings and queens before in the pursuit of their debts, and it's arguable that their contributions are the only reason the Lannisters came out on top in the scramble for the throne. That’s to say nothing of the rumored connection between the Iron Bank and the House of Black and White - home of the Faceless Men.
In Season 7 of Game of Thrones, Cersei Lannister pledges to Tycho Nestoris that she will repay the entirety of the Crown’s debt to the Iron Bank in return for their continued support. This might sound like a good idea, but as Redditor /u/blockpro156 points out in a fan theory, it might also take away the Bank’s only motivation for an ongoing agreement with the Lannisters:
I have a feeling that there's a reason why nobody ever pays the Iron Bank off in a single installment, it's because once you do so, their investment has fully paid off and they're free to look for better investments.
It's better to always maintain a debt with the Iron Bank because that way they're always invested in your continued reign.
In other words, the Iron Bank has a vested interested in keeping those who owe them money in power so they can continue to collect on those debts; once repayment has occurred, that's no longer the case.
Cersei Lannister thinks herself clever, but she’s frighteningly easy to manipulate. When Tycho Nestoris points out that Tywin never managed to pay off his considerable debts, Cersei takes it as confirmation that she’s finally outdone her father - but it might be the exact opposite.
When Tycho says, “I don't think the Iron Bank has ever had a debt of this magnitude repaid in a single installment. I always considered your father a very effective and efficient man, but you appear to be redefining those terms entirely,” it certainly sounds complimentary, but it can also be read as scathing sarcasm.
Given Cersei may have just given away her father’s greatest bargaining chip, it’s probably the latter.