Now, you can justify changes however you like. The mindless zombies somehow destroyed the bridge. A freak storm suddenly destroyed the bridge whatever. It's not really important to me. You destroyed the bridge to tell a better story. If the bridge wasn't destroyed, the PC's would just run away and there would be no adventure.
Just because you justify it through in world actions doesn't make it any less story based. It just makes it a lot more believable.
I'm OK with that. Anything in the game should make sense. Making changes or justifying changes should always make sense.
As a DM, I already accept that I have to make stuff up when the PCs do something unexpected, or I have to invent consequences when the PCs commit a crime. I choose whether anybody sees the PC stab an NPC. I choose whether the cops get called. I choose IF the cops get there in time. I choose how the cops react, and so on.
I therefore choose to take out the bridge, so the scenario isn't too easy. if I do so, it has to make sense. If the PCs circumvent something I thought would happen and prepared for, I choose whether to move it, recycle it, skip it, or have something comletely different happen.
The goal, whenever I do so, is to setup a situation where the PC faces a challenge or has to make a Choice. It's usually safe to assume they're going to choose something to beat the bad guy, but I'm often surprised by the nuance of how they choose to do it. And that's the fun part.
Probably something to note, that when I try to make a story out of it, I'm not planning for long multi-session adventures. I write enough material to cover 4-6 hours. I may have a mental note of big events I'd like to see happen (like having the PCs be at the Battle of the Line versus the elves). But I only write what I need for the next session.
This means I get plenty of room for course correction. I can safely predict what'll happen when the party has shore leave at the island where the monk's dojo is. But I got no clue what they'll do when they learn that a rival dojo has taken something important. Actually, I do know, the monk will go after it, and the PCs will likely help him. But after that, I got no clue how carefal they will be, or how Slicey they'll get.
So I won't say I run a sandbox, but I try to set up a small situation, where I'm dead certain the PCs will get involved, by making it personal to the PCs. After that, the players do whatever they do in that environment, chasing a goal that I knew they'd go after. But the choice was never about "do they fetch the mcguffin", it's "how do they pursue it"
Just like I don't really get a choice to go to work or not (technically I do, but the reality is, I gotta pay a mortgage and feed a family). So that's not really a choice. Nor is the route I take to work a choice. It's not meaningful anyway. But what I do at work is meaningful. How I choose to solve the problems I face either protects my job, or puts me at risk. Those are Choices.