Why destroy Laketown's bridge?

[LadyBrooke]

*Timidly pokes head in*

Looking at the picutre of the one bridge, it seems to me there is one other solution that nobody has mentioned yet. The half closest to the town is very strong and thick looking, but as it gets closer to the shore it gradually becomes smaller. I live about 20 miles from one of the biggest rivers in the country, and there are several old bridges near here. When one of them became too run down to be useable instead of destroying the whole bridge (which would have been costly and dangerous due to the large pillars) they only destroyed the smaller end sections which kept anybody from driving onto it.

Assuming that the bridge in the picture is over a mile long, it seems to me that just destroying the end closest to the shore would have prevented Smaug from walking over it. Laketown might as well be an island if the first fifth of the bridge is missing.

*pulls head back quickly*