I still feel that the future of open is about making it a game mode more people want to play in, trying to make them do it just isn't going to work.
I share this opinion (even though I play mostly in Open). Encouragement to play in Open might be effective (how to achieve this without taking away from those in Solo / Private Groups may be the difficult bit), force is something else again - the old phrase "you can lead a horse to water but you cannot make it drink" springs to mind.
If I was forced to play in Open (bearing in mind that I do mostly) I would not be happy - being locked into a mode is very different from choosing to be there - so I would probably play far away from any of the Powers.... While Frontier, technically, *could* lock all players in Open, no-one can force anyone to actually play the game.
If I had to select a single mode to play in, of the three, I would select Private Groups as they provide both solo play (personal Private Group), group play (obviously) and I would expect that a pseudo-Open Private Group would be created (a quick-join Private Group) that any of the players who had also chosen to be locked into Private Groups could access on a session-by-session basis.
It would be interesting indeed to know how many players would elect to be locked into Open (but we'll probably never know for two reasons: it is extremely unlikely to happen as DBOBE's opinion is that "there is no 'right' way to play the game" and also, Frontier don't release statistics regarding the population of game modes).