My point is that you don't need open polymorphism for the example given, and if you give me another example, I can show you why you don't need it there either -- because you do in fact never need it. It's an XY problem over the set X.

If a programmer is reaching for a library, they want it to solve a concrete task. Nobody wants to subclass your Event type, if I have to do it, you've failed at your job.