They will change things based on feedback and player data when they get time. Diadem and other upcoming exploratory zones are being changed to more objectives-based content in a 3.x series patch, possibly 3.4, and separate matchmaking for gathering classes added. Verminion and other MGS activities will get cross-world lobbies and updated rewards to increase participation. Deep dungeon only just released but will be adjusted and updated like anything else. It's an MMO so things will constantly be re-evaluated and improved over its lifespan with or without a test world.

edit: gotta give S-E props for the latest patch reducing the damage of Raw Destruction in pvp and letting you buy primal mounts for 99 tokens, two things that were requested by many people on the English forums.