An Aztec Capital and Classic MMOs
The short answer is nostalgia. The first moments of playing MMOs are some of the most thrilling and immersive experiences a gamer will ever experience. Moreover, the games we fell in love with have changed in the decades since their release. The long answer accounts for this fact: the old game is a different game.
Sure, nostalgia hypes players up, but I cannot recount how many times I have resubbed to WoW based on nostalgia, only to find myself bored immediately. It can be argued the same thing will happen in Classic WoW. However, a look at other games proves otherwise.
Legacy Servers Across the Genre
In 2012, Runescape underwent an overall. The changes were akin to Cataclysm in WoW. The player base was forced to change their playstyle. While some players did not mind the changes, a large percentage did. After players were polled and a legacy option was found overwhelmingly popular, a legacy server was added. Now, legacy populations are higher than normal servers.
In Rift, a similar legacy server was added. Entitled Rift Prime, the server was only available to subscribers. In Rift Prime, zones are far more populated than the free version of the game, despite the payment aspect.
Similarly, Lord of the Rings Online announced it will have a legendary server. Angmar will be the end-game and players will be restricted to Eriador. Like Rift Prime, only VIP players can access the server.
If You Make it, They Will Pay
This seems to be the mantra for MMO developers. People miss old aspects of the games they once played. They will pay a subscription even if the live game is free to play.
I believe the reason for this is how MMOs are made. Once one is developed and released, the updates after can only deviate from the original product. In the case of WoW, the original products (up until Cataclysm) were when most players were most active. I do not think it is because of nostalgia that players want to experience vanilla MMOs. Rather, the MMO they play now is not the same game.
A Building Upon Foundations
Mexico City was built upon the ruins of Tenochtitlan, the Aztec capital city. It was the most populous city in the entire world at its height, but now its temples are crumbled, its avenues are highways, and its inhabitants are different as well. Yet, underneath Mexico City are the remains of many temple complexes, the last testament to Tenochtitlan.
MMOs are like Mexico City. The vanilla game that drew many people to it has been built upon so that the players no longer recognize what they fell in love with. Only a few scattered, broken systems remain of the base. And while the shiny new features of modern MMOs represent good games in their own right, players have a yearning for past times.