Let’s say you’re right, not everyone figures out twists or even enjoy the same things you do in a movie.  So what’s correct for you, isn’t necessary applicable to others.  So there’s technically no absolute of if spoilers ruin the ending for people.  It’s more that spoilers can take away from the reaction.  Of course like you said, no spoilers does not mean you can’t figure out what will happen, but spoilers definitely means that you won’t be surprised.