What is more likely to happen is that the housing association that helps with the mortgage then offers it to someone else who is in need of this type of social

What is more likely to happen is that the housing association that helps with the mortgage then offers it to someone else who is in need of this type of social housing. The risk becomes less for the housing association each time as more of the principal is paid off. But even when it is paid off they can offer the loan service to new buyers. This happens frequently in new build housing in the UK but it is not without its problems.