First, I want to say, I do agree the world (in SWTOR's case: the galaxy) must evolve. New places must be created to keep the game alive. What I am suggesting is simply, when you launch new patches or expansions, evolve what is currently a vibrant and rich galaxy.
One of my favorite examples of this is the Everquest series. As they released new content they were constantly adding new areas to older zones, but allowed the NPCs in the area to keep to the level of the content introduced. For example, in Everquest II you would do quests at level 50 that would require you to go back to the lower level zones and fight monsters that are level 50.
Why would you want to do this? It is true, we have seen the content before and it could get boring. You would be surprised, there is a lot of nostalgia involved in returning to an older zone, you remember troubles you had with certain enemies or quests, or even mass fights that you had with other players. In World of Warcraft: Cataclysm, when Blizzard revealed that the Horde had taken over Hillsbrad there was an outcry because of how important people thought this area. If you arent familiar with Hillsbrad, it was an area that was a major focus for large scale PVP battles.
Outside of nostalgia there is the point of beauty. A lot of these worlds are very beautiful in their own way. Artist have put hundreds of hours into fine crafting each area of the galaxy, and this reason alone should keep us coming back.