The analysis in the paper shows that once a drug goes off-patent its chance of being repurposed approaches 0%. There is repurposing of on-patent drugs while a

The analysis in the paper shows that once a drug goes off-patent its chance of being repurposed approaches 0%. There is repurposing of on-patent drugs while a monopoly profit can be enforced. That's not the point. The point is that information about which off-patent drugs work in new indications is the definition of a non-excludable and non-rivalrous public good. There is no way to enforce a monopoly profit if people can take the generic drug off-label or have it compounded in a pharmacy. Therefore, there is a tragedy of the commons preventing private investment to unlock this generic drug repurposing innovation (unless someone can come up with a new patented formulation that will not be subject to off-label competition, which is not always possible). A pay for success contract is a solution to this problem. You can also enforce differential pricing for repurposed generic drugs while preventing off-label competition using e-medical records, but this is also another kind of pay for success mechanism (like an advance market commitment or the agreement to pay a minimum order of the Covid vaccines under Operation Warp Speed). However, it might not be politically and commercially feasible to prevent off-label use, especially for generics which are widely prescribed for other off-patent indications.