EVE was never a sandbox game, it keeps being called one but a sandbox must allow you to go around and do what you will without progression being necessary. Since you need SP to unlock a titan for instance EVE is not a sandbox despite how many times people, including CCP state that EVE is a sandbox.
Skill injectors actually moved EVE towards being a sandbox at the cost of becoming play to win if flying a titan in one day was something you wanted. (dont misquote me here, im saying this scenario where a day one player can fly a titan is pay to win, since it cannot be achieved any other way).
Since PLEX can be used for several things i dont see why you think PLEX pulled EVE closer to being something it never was, that of course being a sandbox game.
Instanced game play itself does not define a sandbox game either. The requirement is freedom to move about the game doing as you please, so going into and out of an instance has zero impact on whether a game is a sandbox or not.
Honestly, I would think only fairly simple games could truly be called sandbox games since an form of progression rules the game out: straight out of the box.