Paul - if a person who has bought their home through RtB gets into financial trouble and the home is repossessed, I believe the RtB discount he had received is repayable (for five years after the sale?) when the mortgage company takes possession and disposes of the property. So the taxpayer does not lose out financially. Generally the mortgage provider offers the home back to the social landlord who sold it.
The large number of RtB purchasers who don't get into difficulty and who end up owning their home outright after paying off the mortgage (many do this using redundancy payments, as my parents did, or cash sums taken from their pension fund on retirement) would say that this is not a millstone round their necks until the day they die.
It gives them true security and the ability to enjoy the home they have lived in for decades into their retirement without the threat of a man from the Council coming along and pressurising them to downsize because another family without any social conscience or understanding of contraception "needs" their home more than they do.