any kind of pattern matching without the ability to check for exhaustiveness defeats the safety they provide, which is need to make that comparable in utility and power to inheritance (i.e. so you can actually replace inheritance w/ pattern matching.)

Much like how the compiler assists you when writing subclasseses so that you've implemented all the required methods, real Pattern Matching (F#) gives you the same guarantees from the compiler that you've handled all the possible cases appropriately.

Exhaustiveness checking is what elevates pattern matching to the other side of the expression problem, which is about getting feedback from the compiler to help you write programs.

a glorified switch statement does not help you do that, and so you still have to write subclasses to get compiler feedback.