To make this less abstract, I'm sharing with you an IDR simulation completed for a recent grad with $583K student loan debt. Here's an overview of the various repayment plans:
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Then, the PAYE repayment schedule which includes negative amortization:
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In this simulation, I assume a 40% effective tax rate on any debt cancellation (this sim = $1,066,055). Given the assumptions, including a $1,246/month "tax bomb" investment w/5% rate of return, the effective interest of the student loan is -0.17%.
In response to another comment, IDR borrowers are able to secure other financing. An Endo client and her spouse recently secured $1.5m mortgage having ~ $800k student debt in PAYE. Lender was not selling to Freddie or Fae Mae that made it possible.