It doesn't. We have insolvency rules as well as the doctrine of estoppel introduced in the 1930s. F v B has not stood unchallenged for 150 years.. A frequently referenced case is Central London Property Trust v High Trees House, heard by Lord Denning in 1947.
https://www.bailii.org/ew/cases/EWHC/KB/1946/1.pdf. It was Denning that created the concept of Promissory Estoppel (i.e. a promise acted on to the detriment of the party, should stand, if that party later changes their mind. It would be unfair to trick someone into paying a larger chunk of a debt, borrowing money to do so, and avoiding the protection of bankruptcy, if your intention was to go back for the rest later.
If you act on a promise to you detriment, you can be shielded from the creditor later changing his mind and wanting to enforce his contract.
In SS' example - we have the promise not to declare bankruptcy and an offer of additional funds from other sources. Had the bank changed their mind at a later date - his actions based on their promise would have given him a good chance of defending the claim. Of course, it was always worth a try to come back again later.