Sure you can maybe write a server to handle 3000+ connections, but servers do more than just handle network traffic, they run NPC AI, handle player physics, collision detections, player update code, combat, determining who to send what packets to who, database querys, etc.
That stuff will take its toll on a server or even a set of servers.
Everquest as it is can only support so many players per physical machine. Myabe at the most 250-300 per physical server. (and I think thats pushing it) To acheive even 3000 people in a game world takes a good number of servers, 40+ maybe?
I know when the Shadows of Luclin expansion came out and offered the new "bazaar" feature where players could set up themselves as merchants and sell items and what not while afk, the zone was crashing quite a bit due to the increase in people in the zone. Before the expansion there was maybe 50-80 people in the zone at a time. After the expansion this number shot up to over 300. I have no doubt that sony had to dedicate 1 machine to this zone alone and optimize the code quite a bit to get things stable.