Why destroy Laketown's bridge?

[Sauron the White]

My armour is like tenfold shields, my teeth are swords, my claws spears, the shock of my tail a thunderbolt, my wings a hurricane, and my breath death!

Consider this and tell me why Smaug should be afraid of many armed people attacking him at close range? Smaug's only advantage in the air is speed and mobility. If you consider yourself invincible, this is only little advantage. To the contrary, he is much more deadly when he can engage in closer "combat", which he is unable to do while flying.

Based on that drawing, it looks like Laketown is a crowded place without much room for a large beast like Smaug to maneuver around, let alone quickly. I would think that to box him into a small area where several buildings converge would give attackers on the ground their best chance at him. His speed and agility and overhead advantage is removed and negated. Its like getting a fast moving boxer cornered in a smaller ring where he cannot move around and use his advantages.

Closer combat means his eyes are closer to attackers and how does he protect those? It also lessens the distance between the fired arrows and means they have more force when they hit since they are not going so far out and upwards. I would think the physics of war would be more on the sides of the townies when they can shoot at closer range and perhaps use buildings as shields and protection. But thats just speculation.

I would equate Smaugs boasting to modern day pro wreslters who thump their chests and boast how they will rip out the organs of their opponent while reading from a prepared script. Its part of the show and is all hype and bluster. I think Smaug was not above that sort of thing.