Need explanation on wonders becoming obsolete...

Here's the scenario:
1.) I built the Great Wall wonder
2.) I discovered Engineering first
3.) After what seemed like only 10-15 turns, other civ's began to attack

According to the Civolopedia, a wonder will not become obsolete if you discover the obsoleting tech first. To me in this situation, logic dictates that the Great Wall wonder effect will never obsolete. So what am I missing? Does the effect of a wonder only last for a specific amount of turns? Does the wonder become obsolete in the next era? Does obsoleting a wonder only make it unbuildable for another civ? Sorry if the answer is already on another thread, but I've spent a good amount of time researching this and cannot come up with a conclusive answer. Thanks to all

A wonder will become obsolete as soon as another civ discovers the tech that obsoletes it. I'm not sure of the exact language in the civilopedia, but all it should be saying is that you don't obsolete your own wonders. You can't prevent a wonder from obsoleting by researching the obsoleting tech first, unfortunately.

I guess this is directly opposite of Civ IV where you obsolete your own wonder but other civs don't.