If taxpayer subsidies are to occur, they should be in plain sight for taxpayers to evaluate. Pell Grants are in plain sight and have widespread approval. It is when subsidies get somewhat hidden that people can (rightly) complain. I personally disapprove of IBR because it becomes a hidden subsidy that taxpayers cannot properly evaluate ahead of time.
You want to be an art major, or do social work? That’s all well and good. But their median pay is well understood, and if you want to take educational loans to join a low paying profession, I personally believe you should be prepared to pay back every cent, with market-based interest.
Not doing this leads to perverse incentives. Colleges are encouraged to raise tuition, knowing they will be paid. Students who believe that IBR will apply to them are encouraged to go to expensive colleges, because “why not?”. Taxpayers get stuck with the bill.