So when a user fails to repay a loan, he will expect to receive (multiple) rating(s) from others warning others about lending to this person. The borrower's main economic motivation to repaying a loan he is already late on is for the negative trust to get removed (and potentially be replaced with a neutral rating about the late repayment).

In "the real world" it is very common for people to make late payments late, and to have isolated incidents of repaying a debt past the deadline. Lenders are frequently just as happy to receive a payment on the 31st as they are on the 1st, especially if they receive a late fee of some sort if received on the 31st.

I would opine that as long as the lender is happy, there is no real reason to leave a negative rating if a loan is repaid.