You do know that, with the help of a player from a different server, WoW players are able to go farm on a different server from their own — perhaps going to a low pop PvE server, where they won't see competition for the gathering nodes — and bring back what they earn to their own server, right? And that this doesn't cost a single penny?
Blizzard figured out that the benefits of allowing players to play together, even if they did chose different servers or rulesets at the start, far outweights any perceived benefit from forcing players to stick to their early choice.
(When it comes to gathering nodes I still prefer the GW2 mechanism, though. Nodes are instanced and farming one only makes it vanish for the player that did the farming; others can still see it and farm it themselves.
you cannot manipulate the economy in WoW by hiding from the world with impunity, and there are no solo modes or zero population servers that you can pop in and out of. you must understand that the solo game mode manipulating the open game mode is game-breaking. it is worse than an overpowered module or a class or enemy npc, etc..