There are solutions to priority inversion. This is an old limitation that is now days even taught in school.

Any decent RTOS should have Priority inheritance that should avoid this.

Pointing to this one things as RTOS issue isn't really an accurate portrayal of current RTOS capabilities.

That’s good to know. Indeed, the Pathfinder’s OS (VxWorks) had priority inheritance but it wasn’t enabled on a particular mutex and enabling it was the fix.

Priority inversion had been known about since the 70s. Priority inheritance seems to have first been proposed in 1990:

https://www3.nd.edu/~dwang5/courses/spring18/papers/real-tim... (Priority Inheritance Protocols: An Approach to Real-Time Synchronization)

The Pathfinder engineers were apparently unaware of the priority inheritance option available in VxWorks until they had to debug the issue live from a few hundred million km away.