Actually, I understand where you are coming from.
I'm just saying that this is possible just by expanding the player limit of Mobius, which if what people say is to be believed is already planned, instead of implementing a whole new game mode, which would require more time and work for the same effect. And that time and work could be used to provide a better universal open for everyone, where the PvPers won't be bored out of their skin to the point where they turn to psychopaths and kill those who wish to just do their thing.
However, those who wish to just do their thing also need to understand that what they do has consequences to the game's universe. Consequences that someone else might not like and will want to repel you from doing them completely.
You are correct in that, yes, Frontier could simply keep adding more and more Mobius groups as the demand grows. What, we're at 3 now, right? But where does that stop? Five? Ten? Twenty? What's the threshold that means an open PvE mode is a more effective method than continually expanding the private group alternative that has already well and truly outstripped what private groups were supposed to (or anticipated if you prefer) to accommodate. I would contend that the time to act on introducing an open PvE mode is now, not in two years time, or however many years you like. Indeed, I think it should have been in from day one, as the KS proposed multiple open modes with differing rulesets if I'm not mistaken. If for no other reason than to attempt to reconcentrate the large number of players staying in solo and private groups. Sure, effective punishment mechanics may go some way to doing that, but the psychopaths will still exist despite those consequences - and there'll be many who will maintain a desire to simply avoid open because of the presence of those players (no matter how unlikely it may be that they'd interact with them), irrespective of how effective any punishments may be.