This is probably what the OP means by the reference to "medium-term pain" for authors. In order to transition from up-front lump sum to after-release royalties

This is probably what the OP means by the reference to "medium-term pain" for authors. In order to transition from up-front lump sum to after-release royalties, you have to turn down advances for future books and open a gap in your income stream until the royalties start coming in.