[Rune Son of Bjarne]

If one can reason witha creature like the watcher, then there is nothing speaking against your theory.

The thing is that we have absolutely no reason to think that there was such a creature, if there was then it would have been mentioned. I am absolute confident that if they brought down the bridge in some amazing or elaborate way, then it would have made the story.

Since we are not told about it on has to assume that it was done in a relatively normal and uninteresting way. . . as I said before: Bridges are often thrown down, just because a bridge looks solid it does not mean that it has to take a very long time.

We know that the bridge Turin let build was impossible to destroy in a hurry as it was hughe and made of stone, much unlike the bridge if Esgaroth. Their bridge was wooden and seemingly not amazingly big, Actually the fact that Tolkien points out that the bridge over the Narog is difficult to tear down, sugest that normally throwing down a bridge was not very difficult.