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Pattern matching seems very limited compared to subclass-based method dispatch. Using pattern matching requires you to know every case in advance. But most "standard" OO languages allow subclasses to be created without touching the original base class. This allows injecting new behavior into existing systems. AFAIK, pattern matching alone can't do this.

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That's because it is limited. It's not meant to be a way to provide polymorphism. One of the things that it does provide is the knowledge of every possible case and a reminder from the compiler if you missed one. This is not possible in many OO language.

FP languages have different ways to inject new behavior. You could for example define a function a -> (a -> Int) -> Int. This function now works for any type for which you can also provide the "interface" a -> Int.

You could also define a typeclass Intable, and a function (Intable a) => a -> Int, which will work on any type that implements Intable.

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