At which point, we'll discover that selling such indemnity guarantees as part of software is ludicrously unprofitable and unsustainable, just like the insurance industry has.

There is not a software tool which on its own can defend against ransomware. The most robust defense requires a total system design of "Continuous Restoration", with everything backed up, with backups stored in such a way that they cannot be destroyed, with backups constantly being restored to production, and with periodic human monitoring and manual confirmation that the system is working.