Well, here are some problems with that idea, aside from the extra programming work that would be diverted from more important things like added content:
With no monthly subscriptions, Frontier will need to sell DLC to fund continuing development and the matchmaking servers. This will include walking around stations and landing on planets. It's unreasonable to expect Frontier to make two separate sets of DLC's tailored to those two different versions of the background Galaxy. With everyone sharing one Galaxy, in all our separate playing modes, we can all buy the same DLC content as they roll it out.
If there is a separate version of the game with a locked-in Open-only mode, you'll soon start to hear demands to balance ships, weapons, and entire game content solely for that play style. With one shared Galaxy and a mix of professions in the current game, it means ships, weapons, missions, and Galaxy-wide events are balanced for everyone together, and not a strictly PvP-focused portion of the player base.
And finally, somewhere off in the distance, locked into one of those "pending game content star systems" we can't enter, are Thargoid aliens. Eventually they will start showing up in human space. When they do, we'll probably need the cooperation of the entire player base to deal with it, helping out no matter what mode we're in. It would make no sense to have two separate versions of the Thargoid invasion happening at the same time on two isolated versions of the Galaxy.
We're all in this together, even when we're not entirely together in the same instances. The Thargoids won't care what mode we're in.