In the latest Dev update, FD have again stated that everyone will remain in the same background simulation, no matter their mode of play or which "season" of content they have bought up to. Makes sense to me, and I can't find any way of making a Purest Open Only mode work around that wouldn't require fundamental changes to the game.
And to help highlight this point;
Dev Update (6/8/2015) Last Paragraph said:What we are doing is new in many ways, both technically and in terms of how we are realizing our long term ambitions for Elite Dangerous. As we evolve the game we are trying to give the best value we can to both existing and new players, for the long term benefit of everyone. That’s why we’ve worked hard to keep backwards compatibility for the Elite Dangerous: Horizons season, and are continuing to release updates for ‘season one’ players. Everyone will continue to fly in the same galaxy, and be impacted by, participate in and help to drive the same events.
https://community.elitedangerous.com/node/248