IPP prisoners can't be arbitrarily detained forever. There has to be a reason for the refusal of parole or for a recall to prison. There are clearly significant shortcomings in how IPP prisoners are supported, but it's also clear that a cohort of people who are living with drug addiction, severe mental illness and behavioural problems are going to struggle to stay on the straight and narrow even in ideal circumstances.
https://www.justiceinspectorates.gov.uk/hmiprobation/wp-cont...