What users actually depend on Linux's iron-clad promise to never break binary compatibility? It's curious to me that this issue is considered so sacred and yet I rarely hear the rationale or actual user stories of people who want to run 20-year-old userland on a new kernel.

In particular, note that the promise of kernel binary compatibility does not guarantee that an old binary will run on a modern Linux distro unless the binary is statically linked. Most user-space libraries bump their major version number every so often, so it's unlikely that the required .so's for a very old binary will be present on a new system.