Of course nothing should be inflexible, but I'm just saying there's no reason for every campaign to be derailed completely. If your players are actively going against the scenario something's gone wrong. And yeah, if you give them the opportunity to do something, be prepared for it to happen and roll with the consequences.
Unless you play low level John Does every time, characters should know stuff about the world they've lived in their whole lives. And if none of that ties into the scenario are you playing a campaign or in a sandbox?
I'm so glad someone's mentioned CrossCode! Such a wonderful experience from beginning to end. The world really feels alive with every inch of a mal being used to either enhance the story or hide a puzzle! I loved seeing chests and figuring out how to get to them across several maps.
I'm really looking forward to their next project, Alabaster Dawn. I hope it's just as good!