Read chronicles again, little horde Fanboy.
its explains that orgrimm decides to create a fleet because the bridge was a bottleneck.
Not to say that the bridge could be torn down
And anyway, at the end of second war, horde could not destroy a single capital. I doubt that even by conquering lordaeron, they could defeat the other kingdoms, having lost so much.
Accept it. Horde only had one chance to conquer lordaeron, thanks to Neltharion's manipulations.
Without Neltharion's help the horde would have been extinguished before the second war began.