True. And yet I can look up Mousetrap on Wikipedia and learn who "dunnit" without sitting through the play. Will I never go see The Mousetrap now? I still might. Because knowing how it ends is only part of the experience of seeing the play. Surely there is more to a story than the number and type of plot twists it contains.
Sure, that's your individual choice. But that doesn't make it hard to comprehend why the writer or performers of a mystery play, out of all possible genres, would ask their audience not to spoil the ending.
After all, Wikipedia didn't exist when the play was written. We live in an age where we're inundated with information and have answers at our fingertips, and that changes the way we engage with the world. The current generation often appreciates having instant answers to any question. But mysteries are written by and for people who enjoy the suspense of not knowing and the challenge of deducing an answer rather than just looking it up. Maybe you're not part of that audience, but that audience obviously does exist, or mysteries wouldn't be such a prominent genre.
Is The Empire Strikes Back unwatchable now that "Luke I am your father" (really "No. I am your father") is a pop culture meme?
No, but it's not a mystery. Not knowing the answers is critical to the mystery genre. It's what the name means!
Sure, it can be possible to enjoy a mystery if you know the outcome, but most fans of mystery want
to be in the dark so they have the chance to figure that out for themselves.