Why destroy Laketown's bridge?

[Sauron the White]

I am sorry ... when I read the post I thought that the main point by Alfirin was

taking down the bridge might have been the only way to keep the Men of dale around to fight, by giving them no choice. (much like Julius Caesar burned the bridges behind his troops during the gallic wars to give them no ability to reatrat

.

If the only way off Laketown was that bridge I would agree with that point.
But it was not the only way off Laketown. And the people who ordered the destruction of the bridge knew that fact very well since they lived there.
Which is why I responded with

Yes but many left in droves via boat.

I think that directly adresses the main point by Alfirin. If I am still failing to see that point I would welcome further explaination.