The best DMs are the ones that can adapt and come up with stuff on the fly. The only guaranteed thing is that the players are idiots and will try and punch a NPC in the tavern on day 1… throwing your well planned story out the window.

The best games I have ever ran my players would go major off-plot like spending the entire session befriending a rando NPC I didn’t even have a statblock for or turning a random wilderness encounter into a sessions long war against a pack of worgs.

It took me a while to get over all the lost planning time, and it always turned out to be worth it to let my players have reign.