Book value accounting only works if you have matching regulatory or matching duration capital for the original loan.

Mortgage banks drove the UK bankrupt cap in hand to the IMF in 73 with far far less of a rates delta than is happening now.

Edit : there was a similar breakdown of society as is happening already today almost everywhere , back in 73 in the UK. The main difference is that the banks managed to put much more of the pain onto the common man since then.