The psychological burden of an indefinite sentence is undoubtedly an obstacle to rehabilitation, as are the wider issues of understaffing and overcrowding in the prison estate, but let's not kid ourselves. IPP sentences were created specifically to deal with prisoners who habitually reoffend on release. These are people who were on a revolving door of offending and imprisonment before their IPP sentence, and who would almost certainly be stuck in that cycle again if their IPP sentence was quashed.
IPPs are unjust and the current government will almost certainly retroactively resentence most or all IPP prisoners within this parliament, but the underlying problem is still there. We could definitely do a better job of rehabilitating this cohort, but even the best-funded and most liberal systems have had fairly meagre success in actually breaking the cycle of reoffending; the much-heralded low recidivism rates in Scandinavia are substantially a reporting artifact rather than a meaningful difference in outcomes.