Manetheren was an end of Breaking society unlike most of the modern nations. It was tucked deep into a mountain range, prioritizing protection from attack over trade. But the society would have had to fail eventually or so significantly readjusted from defense to trade for the country to survive, a very unlikely action.

Most modern countries and capitals are on navigable waters.

Naw, Manetheren at its peak had a whole system of paved roads and bridges. They traded easily and often with other Nations. They were a wealthy trade Nation. Most of those bridges were destroyed on purpose during the Trolloc Wars to slow them down.

You have it backwards, they adjusted from trade to defense during the Trolloc Wars.

The Fall of Manetheren was the beginning of the end of the Trolloc Wars. The Shadow threw the majority of its resources into destroying Manetheren and it cost them so dearly accomplishing this that they never took another forward step after, only backwards as they were pushed back into the Blight.