Without relinquishing claim to the account it can't be truly considered being held in escrow awaiting loan repayment. The reason why this model works out to minimize lender risk is regardless of if the borrowers disappears or meets their fate or whatever outcome may be the loan will get repaid.
Perhaps your method would work but on a fundementals basis it's much easier to explain to a user that the accounts keys and therefore ownership are changed outright rather than trying to explain the subtle nuances of key threshholds and such.
I do thoroughly like your idea though..! I might experiment with something along these lines out of curiosity just to see if the 100% intended effects of key changing filter over to key threshold modifications.